build: Remove unused import json

One of my patches added an unused import for json in vm.py.  Remove it
and fix the build.  My apologies.

Change-Id: I6d33c81c8a29d2facc08b1f4c573b2d87d2064dc
Signed-off-by: Adam Litke <alitke@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/28660
Reviewed-by: Dan Kenigsberg <danken@redhat.com>