blockdev: Improve message for orphaned -drive

blockdev: Improve message for orphaned -drive

We warn when a -drive isn't supported by the machine type (commit
a66c9dc):

    $ qemu-system-x86_64 -S -display none -drive if=mtd
    Warning: Orphaned drive without device: id=mtd0,file=,if=mtd,bus=0,unit=0

Improve this to point to the offending bit of configuration:

    qemu-system-x86_64: -drive if=mtd: warning: machine type does not support if=mtd,bus=0,unit=0

Especially nice when it's hidden behind -readconfig foo.cfg:

    qemu-system-x86_64:foo.cfg:140: warning: machine type does not support if=mtd,bus=0,unit=0

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <1487153147-11530-7-git-send-email-armbru@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>