vfio-pci: Fix remove path locking

Locking both the remove() and release() path results in a deadlock
that should have been obvious.  To fix this we can get and hold the
vfio_device reference as we evaluate whether to do a bus/slot reset.
This will automatically block any remove() calls, allowing us to
remove the explict lock.  Fixes 61d792562b53.

Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Cc: stable@vger.kernel.org	[3.17]