xen/blkfront: correctly detach a disk with active users

Call disk_gone when the backend switches to the "Closing" state and blkfront
still has pending users. This allows the disk to be detached, and will call
into xbd_closing by itself when the geom layout cleanup has finished.

Reported by: bapt
Tested by: manu
Reviewed by: bapt
Sponsored by: Citrix Systems R&D
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D10772