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

To accommodate the call to the callback function from
__xen_pcibk_publish_pci_roots(), which so far dropped and the re-
acquired the lock without checking that the list didn't actually
change, convert the code to use a mutex instead (observing that the
code taking the lock won't ever get called from non-sleepable
context).

As a result, drop the bogus use of list_for_each_entry_safe() and
remove the inappropriate dropping of the lock.

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