xenapi: Disable VDI size check when root_gb=0

Nova uses root_gb=0 to indicate that we should use the image as-is without
trying to resize the root disk. Currently the xenapi virt-driver doesn't
properly implement this behavior.

This patch gets us a step closer by disabling the VDI size check when
root_gb=0. We still need to ensure that the disk-resize code handle root_gb=0
as well.

References bug 1155113

Change-Id: If91963e079e197a1315c48b8643f3419b42b0b2c
Merge "xenapi: Disable VDI size check when root_gb=0"