block: vhdx - add header update capability.

This adds the ability to update the headers in a VHDX image, including
generating a new MS-compatible GUID.

As VHDX depends on uuid.h, VHDX is now a configurable build option.  If
VHDX support is enabled, that will also enable uuid as well.  The
default is to have VHDX enabled.

To enable/disable VHDX:  --enable-vhdx, --disable-vhdx

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