hmp: Allow using a qdev id in block_set_io_throttle

The QMP version of this command can take a qdev ID since 7a9877a0,
but the HMP version is still using the deprecated block device name so
there's no way to refer to a block device added like this:

  -blockdev node-name=disk0,driver=qcow2,file.driver=file,file.filename=hd.qcow2
  -device virtio-blk-pci,id=virtio-blk-pci0,drive=disk0

This patch works around this problem by using the specified name as a
qdev ID if the block device name is not found.

Signed-off-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>