SCTX-1473 [HFX-1077]: Unable to start VM by VM_REQUIRES_GPU. Multiple PCI consumed

by a single VM.
The GPU code in the start_paused function selects a GPU to be passed
through during a VM start. This does not take into account the fact that
xapi has already taken care of the PCI devices attached to the VM before
reboot.
Hence during a VM reboot the VM will get assigned a different PCI device
other than the one it was holding prior to the reboot.
Signed-off-by: Akshay <akshay.ramani@citrix.com>