virtio disk hot-unplug: drive_del after device_del

virtio disk hot-unplug: drive_del after device_del

we need to remove drive after device has correctly by remove.

With this patch:

Windows : if drive is locked/"mounted", the hot-unplug fail and disk is accessible (100% safe)

Linux: without hotplug module : the hot-unplug fail and disk is accessible (100% safe)

Linux : with hotplug module : the hot-unplug work, but no verification is done is the drive is mounted

Signed-off-by: Alexandre Derumier <aderumier@odiso.com>