blockdev: Make orphaned -drive fatal

Block backends defined with "-drive if=T" with T other than "none" are
meant to be picked up by machine initialization code: a suitable
frontend gets created and wired up automatically.

If machine initialization code doesn't comply, the block backend
remains unused.  This triggers a warning since commit a66c9dc7, v2.2.0.
Drives created by default are exempted; use -nodefaults to get rid of
them.

Turn this warning into an error.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <1487153147-11530-8-git-send-email-armbru@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>