qemu-iotests: Test unplug of -device without drive

This caused an assertion failure until recently because the BlockBackend
would be detached on unplug, but was in fact never attached in the first
place. Add a regression test.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
2 files changed