qdev: support to get a device firmware path directly

qdev: support to get a device firmware path directly

commit 6b1566c (qdev: Introduce FWPathProvider interface) did a
good job for supproting to get firmware path on some different
architectures.

Moreover further more, we can use the interface to get firmware
path name for a device which isn't attached a specific bus,
such as virtio-bus, scsi-bus etc.

When the device (such as vhost-scsi) realize the TYPE_FW_PATH_PROVIDER
interface, we should introduce a new function to get the correct firmware
path name for it.

Signed-off-by: Gonglei <arei.gonglei@huawei.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>