XenAPI: Allow 10GB overhead on VHD file check size

_check_vdi_size in vm_utils.py does not account for VHD overhead. Builds
will fail if on disk VHD file usage is even slightly greater than the
root_gb partition allocation size to the VM.

With VHDs, VM root disks usually come from two-file chains (one, if a
snapshot has never been taken or it was not a fast copy build). There is
a mild file format overhead involved and additionally, once a block has
been written to, the space is never freed or compressed from the VHD
file. The two VHD files in the chain may have overlapping blocks adding
to the total amount of overhead over the actual utilization. This can
result in a VM's partition internally not even being full, with the
external VHD File chain being even just a few MB larger than the allowed
size. Building from an image of such a VM will fail due to this check
expecting external/on-disk usage to never be greater than the internal
partion size granted.

This patch adds a 10GB tolerance which should fix almost all cases of
build failures due to this bug.

Fixes: bug #1216042

Change-Id: I69f47351fe83e54775e0147a27450c21196f9bfd
Merge "XenAPI: Allow 10GB overhead on VHD file check size"