block: Adds mirroring tests for resized images

This test verifies two mirroring issues are fixed with resized images:

 * sync='top' creates an image that is the proper size
 * sync='full' doesn't cause an assertion failure and crash qemu
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
