vm: janitorial: replace file() with open()

As per docs, open() is preferred to file():
http://docs.python.org/2/library/functions.html#file

moreover, file() was removed in python3:
http://docs.python.org/release/3.0/whatsnew/3.0.html#builtins

Change-Id: I39c604b53e812a838542c94349cb64c272bea552
Signed-off-by: Francesco Romani <fromani@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/22034
Reviewed-by: Antoni Segura Puimedon <asegurap@redhat.com>
Reviewed-by: Assaf Muller <amuller@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>