Fix incorrect root partition size and compatible volume name

When using rbd backend, it needs to resize volume size if size presents. Else
we will get wrong image size.

Rbd volume always uses uuid as the main pattern of volume name, now change to
it is necessary.

This patch also include a follow up fix of the original patch.
https://review.openstack.org/#/c/92512/

Fix bug 1219658

(cherry picked from commit 1b66a47b5f4cbcdf1a9a1ec38532474588a3fee2)

Conflicts:
	nova/virt/libvirt/imagebackend.py

Change-Id: I3dc8d639b066eac35d190b8d4687c41c9f4d5824
Merge "Fix incorrect root partition size and compatible volume name" into stable/havana