In order to issue requests on an existing BlockBackend with the
'qemu-io' HMP command, allow specifying the BlockBackend not only with a
BlockBackend name, but also with a qdev ID/QOM path for a device that
owns the (possibly anonymous) BlockBackend.

Because qdev names could be conflicting with BlockBackend and node
names, introduce a -d option to explicitly address a device. If the
option is not given, a BlockBackend or a node is addressed.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>