PCI: hv: Change pci_protocol_version to per-hbus

A VM can have multiple Hyper-V hbus. It's incorrect to set the global
variable 'pci_protocol_version' when *every* hbus is initialized in
hv_pci_protocol_negotiation(). This is not an issue in practice since
every hbus should have the same value of hbus->protocol_version, but
we should make the variable per-hbus, so in case we have busses
with different protocol versions, the driver can still work correctly.

Signed-off-by: Dexuan Cui <decui@microsoft.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Michael Kelley <mikelley@microsoft.com>