block: Explicitly specify 'unsigned long long' for VHDX 64-bit constants

On 32-bit hosts, some compilers will warn on too large integer constants
for constants that are 64-bit in length.  Explicitly put a 'ULL' suffix
on those defines.

Reported-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Jeff Cody <jcody@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>