PCI: hv: Convert remove_lock to refcount

Use refcount instead of atomic for the reference counting
on bus. Refcount is safer because it handles overflow correctly.

Signed-off-by: Stephen Hemminger <sthemmin@microsoft.com>
[lorenzo.pieralisi@arm.com: updated commit subject]
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>