Don't import guestfs directly

[ChangBo Guo(gcb)]

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.

[Sahid Orentino (sahid-ferdjaoui)]

There is something strange, I don't understand why we try to import guestfs and then set it to None to import it locally.
I confirm it because I feel there is something that can be improve.

[ChangBo Guo(gcb) (glongwave)]

This is code review : https://review.openstack.org/#/c/80741/

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/80741
Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=4ecc1c52dbe02d5c4211d3936e2eec57dcc22de3
Submitter: Jenkins
Branch: master

commit 4ecc1c52dbe02d5c4211d3936e2eec57dcc22de3
Author: ChangBo Guo(gcb) <email address hidden>
Date: Sat Mar 15 10:17:24 2014 +0800

    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