blockdev: Orphaned drive search

When users use command line options like -hda, -cdrom,
or even -drive if=ide, it is up to the board initialization
routines to pick up these drives and create backing
devices for them.

Some boards, like Q35, have not been doing this.
However, there is no warning explaining why certain
drive specifications are just silently ignored,
so this function adds a check to print some warnings
to assist users in debugging these sorts of issues
in the future.

This patch will not warn about drives added with if_none,
for which it is not possible to tell in advance if
the omission of a backing device is an issue.

A warning in these cases is considered appropriate.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Message-id: 1412187569-23452-2-git-send-email-jsnow@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>