partially fix the bug of live-migration with config drive
partially fix the bug of live-migration with config drive

In current version of nova, live-migration with config drive on
local disk is forbbiden due to the bug of libvirt of copying readonly
disk.

However, if we use vfat as the format of config drive,
the function of live-migration works well.
In this patch, we re-open the function of live-migration to admin users.
Notice you should add 'config_drive_format = vfat' in nova.conf explicitly.
This patch doesn't solve the problem fundamentally which need further
efforts, but offers a simple and feasible solution to user.

Change-Id: If7ab4f35d78bb95b0668e25b5cdd202d64feb288

luogangyi/bcec-nova,luogangyi/bcec-nova