qdev-monitor: improve error message when alias device is unavailable

When trying to instantiate an alias that points to a device class that
doesn't exist, the error message looks like qemu misunderstood the
request:

$ s390x-softmmu/qemu-system-s390x -device virtio-gpu
qemu-system-s390x: -device virtio-gpu: 'virtio-gpu-ccw' is not a valid
device model name

Special-case the error message to make it explicit that alias
expansion is going on:

$ s390x-softmmu/qemu-system-s390x -device virtio-gpu
qemu-system-s390x: -device virtio-gpu: 'virtio-gpu' (alias
'virtio-gpu-ccw') is not a valid device model name

Suggested-By: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Sascha Silbe <silbe@linux.vnet.ibm.com>
Message-Id: <1455831854-49013-2-git-send-email-silbe@linux.vnet.ibm.com>
Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
