get volume virtual size correctly

For some format volume(vmdk, etc.), 'qemu-img info' output looks like:
$ qemu-img info /tmp/vmdk-v4.img
image: /tmp/vmdk-v4.img
file format: vmdk
virtual size: 1.0M (1048576 bytes)
disk size: 12K
cluster_size: 65536
Format specific information:
    cid: 1418280048
    parent cid: 4294967295
    create type: monolithicSparse
    extents:
        [0]:
            virtual size: 1048576
            filename: /tmp/vmdk-v4.img
            cluster size: 65536
            format:

and only the first line of 'virtual size' is what we want.

Signed-off-by: Yanbing Du <ydu@redhat.com>