block: Accept device model name for x-blockdev-remove-medium

In order to remove the need for BlockBackend names in the external API,
we want to allow qdev device names in all device related commands.

This converts x-blockdev-remove-medium to accept a qdev device name.

As the command is experimental, we can still remove the 'device' option
that uses the BlockBackend name. This requires some test case changes
and is left for another series.

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