Run check_blktap_sharing() only when a disk is added to guest with blktap.
A check_blktap_sharing() doesn't have to be run when a disk is removed.
Signed-off-by: Hirofumi Tsujimura <tsujimura.hirof@jp.fujitsu.com>
Signed-off-by: Takanori Kasai <kasai.takanori@jp.fujitsu.com>