xen/pciback: use mutex rather than spinlock in vpci backend

Similar to the "xen/pciback: use mutex rather than spinlock in passthrough backend"
this patch converts the vpci backend to use a mutex instead of
a spinlock. Note that the code taking the lock won't ever get called
from non-sleepable context

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>