CA-123007: Hold a mutex inside Device.PCI.unbind_from_nvidia

If multiple threads are trying to unbind different devices in parallel,
there is a chance that some of the nodes under /proc/driver/nvidia/ will
disappear after a thread has retrieved the list of nodes but before that
thread has accessed their contents.

Holding a mutex for the duration of the (find device,lock device,unbind)
sequence means this should no longer happen.

Imported from xen-api.git ce643b2c09116674af98ed99aa0d9c9384d3c724

Signed-off-by: John Else <john.else@citrix.com>
Imported-by: John Else <john.else@citrix.com>