Goal: 

The active image should have the virtual size of 2GB, corresponding to the size in Cinder's database.

Acceptance Criteria:

Steps to reproduce:
1) Create a new nfs volume with a size of 1G. `qemu-img info` will match the size in Cinder.

$ qemu-img info -f raw volume-87790836-8b53-416d-8a04-b5ca5bef53ad
image: volume-87790836-8b53-416d-8a04-b5ca5bef53ad
file format: raw
virtual size: 1 GiB (1073741824 bytes)
disk size: 21.5 MiB

2) Create a snapshot of the nfs volume
3) resize the NFS volume to 2G
4) volume list shows new volume size of 2GB but the active nfs image still have the virtual size set to 1G (should be updated to 2G as well)

$ qemu-img info -f qcow2 volume-87790836-8b53-416d-8a04-b5ca5bef53ad.55514be7-e1b0-408e-b747-a612c22d2700
image: volume-87790836-8b53-416d-8a04-b5ca5bef53ad.55514be7-e1b0-408e-b747-a612c22d2700
file format: qcow2
virtual size: 1 GiB (1073741824 bytes)
disk size: 196 KiB
cluster_size: 65536
backing file: volume-87790836-8b53-416d-8a04-b5ca5bef53ad
backing file format: raw
Format specific information:
    compat: 1.1
    compression type: zlib
    lazy refcounts: false
    refcount bits: 16
    corrupt: false
    extended l2: false