block: qemu-iotests for vhdx, read sample dynamic image

This adds the VHDX format to the qemu-iotests format, and adds
a read test.  The test reads from an existing sample image, that
was created with Hyper-V under Windwos Server 2012.

The image file is a 1GB dynamic image, with 32MB blocks.

The pattern 0xa5 exists from 0MB-33MB (past a block size boundary)

The pattern 0x96 exists from 33MB-66MB (past another block boundary,
and leaving a partial blank block)

From 66MB-1024MB, all reads should return 0.

Although 1GB dynamic image with 66MB of data, the bzip2'ed image
file size is only 874 bytes.

This also adds in the IMGFMT_GENERIC flag, so r/o images can be
tested (e.g. ./check -vhdx) without failing tests that assume
r/w support.

Signed-off-by: Jeff Cody <jcody@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
