Don't import library guestfs directly

Library guestfs is not in requirements file, but is imported directly
in nova/virt/disk/vfs/guestfs.py. That conflicts with global variable
guestfs, leads out one more time import and shadow issue, so remove it
from import group.

Closes-Bug: #1293298

Change-Id: I55530ed682ebf449673759ab6127bcd28cb92594
Merge "Don't import library guestfs directly"