vfio: cleanup vfio_get_device error path, remove vfio_populate_device callback

vfio: cleanup vfio_get_device error path, remove vfio_populate_device callback

Now that vfio_put_base_device is called unconditionally at instance_finalize
time, it can be called twice if vfio_populate_device fails.  This works
but it is slightly harder to follow.

Change vfio_get_device to not touch the vbasedev struct until it will
definitely succeed, moving the vfio_populate_device call back to vfio-pci.
This way, vfio_put_base_device will only be called once.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>