hmp: Use blockdev-change-medium for change command Use separate code paths for the two overloaded functions of the 'change' HMP command, and invoke the 'blockdev-change-medium' QMP command if used on a block device (by calling qmp_blockdev_change_medium()). Signed-off-by: Max Reitz <mreitz@redhat.com> Reviewed-by: Eric Blake <eblake@redhat.com> Reviewed-by: Kevin Wolf <kwolf@redhat.com> Signed-off-by: Kevin Wolf <kwolf@redhat.com>