blockdev: Use BlockBackend to resize in qmp_block_resize()

In order to be able to do permission checking and to keep working with
the BdrvChild based bdrv_truncate() that this involves, we need to
create a temporary BlockBackend to resize the image.

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