blockdev: Replace "undefined error" in qmp_block_resize

We have an errno value that can be displayed, so we should just do that.
An easy way to reproduce this case is to resize a raw image to a size
that is too large for the host file system.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>