qemu-img: Clarify about relative backing file options

qemu-img: Clarify about relative backing file options

It's not too surprising when a user specifies the backing file relative
to the current working directory instead of the top layer image. This
causes error when they differ. Though the error message has enough
information to infer the fact about the misunderstanding, it is better
if we document this explicitly, so that users don't have to learn from
mistakes.

Signed-off-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Jeff Cody <jcody@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>