blockdev: Introduce DriveInfo.enable_auto_del

blockdev: Introduce DriveInfo.enable_auto_del

BlockDriverStates shouldn't be affected by an unplugged guest device,
except if created with the legacy -drive command line option or the
drive_add HMP command.

Make the automatic deletion as well as cancelling of jobs conditional on
an enable_auto_del boolean that is only set in drive_init().

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Wenchao Xia <xiawenc@linux.vnet.ibm.com>