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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:

def service_start(host,
                  username,
                  password,
                  service_name,
                  protocol=None,
                  port=None,
                  host_names=None):
    '''
    Start the named service for the given host or list of hosts.

    host
        The location of the host.

    username
        The username used to login to the host, such as ``root``.

    password
        The password used to login to the host.

    service_name
        The name of the service for which to set the policy. Supported service names are:
          - DCUI
          - TSM
          - SSH
          - lbtd
          - lsassd
          - lwiod
          - netlogond
          - ntpd
          - sfcbd-watchdog
          - snmpd
          - vprobed
          - vpxa
          - xorg

    protocol
        Optionally set to alternate protocol if the host is not using the default
        protocol. Default protocol is ``https``.

    port
        Optionally set to alternate port if the host is not using the default
        port. Default port is ``443``.

    host_names
        List of ESXi host names. When the host, username, and password credentials
        are provided for a vCenter Server, the host_names argument is required to tell
        vCenter the hosts for which to start the service.

        If host_names is not provided, the service will be started for the ``host``
        location instead. This is useful for when service instance connection information
        is used for a single ESXi host.

    CLI Example:

    .. code-block:: bash

        # Used for single ESXi host connection information
        salt '*' vsphere.service_start my.esxi.host root bad-password 'ntpd'

        # Used for connecting to a vCenter Server
        salt '*' vsphere.service_start my.vcenter.location root bad-password 'ntpd' \
        host_names='[esxi-1.host.com, esxi-2.host.com]'
    '''
    service_instance = salt.utils.vmware.get_service_instance(host=host,
                                                              username=username,
                                                              password=password,
                                                              protocol=protocol,
                                                              port=port)
    host_names = _check_hosts(service_instance, host, host_names)
    valid_services = ['DCUI', 'TSM', 'SSH', 'ssh', 'lbtd', 'lsassd', 'lwiod', 'netlogond',
                      'ntpd', 'sfcbd-watchdog', 'snmpd', 'vprobed', 'vpxa', 'xorg']
    ret = {}

    # Don't require users to know that VMware lists the ssh service as TSM-SSH
    if service_name == 'SSH' or service_name == 'ssh':
        temp_service_name = 'TSM-SSH'
    else:
        temp_service_name = service_name

    for host_name in host_names:
        # Check if the service_name provided is a valid one.
        # If we don't have a valid service, return. The service will be invalid for all hosts.
        if service_name not in valid_services:
            ret.update({host_name: {'Error': '{0} is not a valid service name.'.format(service_name)}})
            return ret

        host_ref = _get_host_ref(service_instance, host, host_name=host_name)
        service_manager = _get_service_manager(host_ref)
        log.debug('Starting the \'%s\' service on %s.', service_name, host_name)

        # Start the service
        try:
            service_manager.StartService(id=temp_service_name)
        except vim.fault.HostConfigFault as err:
            msg = '\'vsphere.service_start\' failed for host {0}: {1}'.format(host_name, err)
            log.debug(msg)
            ret.update({host_name: {'Error': msg}})
            continue
        # Some services are restricted by the vSphere License Level.
        except vim.fault.RestrictedVersion as err:
            log.debug(err)
            ret.update({host_name: {'Error': err}})
            continue

        ret.update({host_name: {'Service Started': True}})

    return ret