Starts a packet capture.

        :param adapter_number: adapter number
        :param output_file: PCAP destination file for the capture

def start_capture(self, adapter_number, output_file):
        """
        Starts a packet capture.

        :param adapter_number: adapter number
        :param output_file: PCAP destination file for the capture
        """

        try:
            adapter = self._ethernet_adapters[adapter_number]
        except IndexError:
            raise QemuError('Adapter {adapter_number} does not exist on QEMU VM "{name}"'.format(name=self._name,
                                                                                                 adapter_number=adapter_number))

        nio = adapter.get_nio(0)

        if not nio:
            raise QemuError("Adapter {} is not connected".format(adapter_number))

        if nio.capturing:
            raise QemuError("Packet capture is already activated on adapter {adapter_number}".format(adapter_number=adapter_number))

        nio.startPacketCapture(output_file)

        if self.ubridge:
            yield from self._ubridge_send('bridge start_capture {name} "{output_file}"'.format(name="QEMU-{}-{}".format(self._id, adapter_number),
                                                                                               output_file=output_file))

        log.info("QEMU VM '{name}' [{id}]: starting packet capture on adapter {adapter_number}".format(name=self.name,
                                                                                                       id=self.id,
                                                                                                       adapter_number=adapter_number))