block/vpc: tests for auto-detecting VPC and Hyper-V VHD images

This tests auto-detection, and overrides, of VHD image sizes created
by Virtual PC, Hyper-V, and Disk2vhd.

This adds three sample images:

hyperv2012r2-dynamic.vhd.bz2 - dynamic VHD image created with Hyper-V
virtualpc-dynamic.vhd.bz2    - dynamic VHD image created with Virtual PC
d2v-zerofilled.vhd.bz2       - dynamic VHD image created with Disk2vhd

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