hmp: Add read-only-mode option to change command

Expose the new read-only-mode option of 'blockdev-change-medium' for the
'change' HMP command.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
