qga-win: changing --retry-path option behavior

qga-win: changing --retry-path option behavior

Currently whenever the qemu-ga's service doesn't find the virtio-serial
the run_agent() loops in a QGA_RETRY_INTERVAL (default 5 seconds)
intervals and try to restart the qemu-ga which causes a synchronous loop.
Changed to wait and listen for the serial events by registering for
notifications a proper serial event handler that deals with events:
  DBT_DEVICEARRIVAL        indicates that the device has been inserted and
                           is available
  DBT_DEVICEREMOVECOMPLETE indicates that the devive has been removed
Which allow us to determine when the channel path is available for the
qemu-ga to restart.

Signed-off-by: Bishara AbuHattoum <bishara@daynix.com>
Signed-off-by: Sameeh Jubran <sameeh@daynix.com>
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>