Opens the process for reading.

def Open(self):
    """Opens the process for reading."""

    self.h_process = kernel32.OpenProcess(
        PROCESS_VM_READ | PROCESS_QUERY_INFORMATION, 0, self.pid)
    if not self.h_process:
      raise process_error.ProcessError(
          "Failed to open process (pid %d)." % self.pid)

    if self.Is64bit():
      si = self.GetNativeSystemInfo()
      self.max_addr = si.lpMaximumApplicationAddress
    else:
      si = self.GetSystemInfo()
      self.max_addr = 2147418111

    self.min_addr = si.lpMinimumApplicationAddress