vfio: fix device unplug when several devices per group

vfio: fix device unplug when several devices per group

VFIO allows a secure way of assigning devices to user space and those
devices which can not be isolated from other ones are set in same VFIO
group. Releasing or unplugging a device should be aware of remaining
devices is the same group for avoiding to close such a group.

Fixes: 94c0776b1bad ("vfio: support hotplug")
Signed-off-by: Alejandro Lucero <alejandro.lucero@netronome.com>
Acked-by: Anatoly Burakov <anatoly.burakov@intel.com>