hyperv/pcib: use the device serial number as PCI domain

Currently the PCI domain is initialized with the instance GUID in
vmbus_pcib_attach(). It turns out the GUID can change across VM reboot,
while some users want a persistent value for PCI domain. The solution is
that we can change to use the device serial number, which starts with 1
and is unique within a VM.

Obtained from: Haiyang Zhang
MFC after: 1 day
Sponsored by: Microsoft