qdev-monitor: Improve error message for -device nonexistant

Once upon a time, the error message was:

    qemu: -device nonexistant: Device "nonexistant" not found.  Try -device '?' for a list.

But progress marches on, and conversion to QError (commit 0204276)
changed it into:

    Invalid parameter 'driver'
    Try with argument '?' for a list.

Progress didn't stop there, of course.  After a couple of iterations,
we arrived at the current message (commit 6acbe4c):

    qemu: -device nonexistant: Parameter 'driver' expects device type

Mission accomplished: this is complete mush.

We've since abandoned our quest for "rich" error objects, fortunately
before it turned all error messages into mush.  Time to undo the
damage to this one.  Make it:

    qemu: -device nonexistant: nonexistant is not a valid device model name

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>
