On Control+C or SIGTERM, initiate a hot unplug of the device

If the guest co-operates then the device will shutdown and the process
will exit.

If the guest fails to co-operate then the process will hang around;
you'll have to SIGKILL it and this will effectively 'force unplug'
the device from the guest.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>