Fix the bug of can't get the desired image info

Fix the bug of can't get the desired image info

When we use 'qemu-img info' command to get image info, some kinds
of image may show an 'unavailable' disk size. For example:

$qemu-img info sheepdog:volume-1af1afa0-7820-4063-9b90-4fa6161a74a9
image: sheepdog:volume-1af1afa0-7820-4063-9b90-4fa6161a74a9
file format: raw
virtual size: 10G (10737418240 bytes)
disk size: unavailable

$qemu-img info -f rbd rbd:data/foo
image: rbd:data/foo
file format: rbd
virtual size: 10G (10737418240 bytes)
disk size: unavailable
cluster_size: 4194304

The current code didn't consider 'unavailable' size, and it will
raise a ValueError in this case. This patch fixed this bug by
adding the judgment of 'unavailable' size.

Change-Id: Ic5f3b80e172baaafc07432319f8aa24bf117b392
Closes-Bug: #1516083