block: qemu-iotests for vhdx, add write test support

This removes the IMGFMT_GENERIC blocker for read-only, so existing
iotests run read/write tests for vhdx images created by qemu-img (e.g.
tests 001, 002, 003).

In addition, this updates the sample image test for the Hyper-V
created image, to verify we can write it as well.

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