virt.qemu_storage: Always check the image by qemu-img info, if glusterfs or iscsi used.

As os.path.exists() do not support gluster://IP:port/volume/image.qcow2 and
iscsi://IP:port/iqn.*:target0/$lun

Signed-off-by: Feng Yang <fyang@redhat.com>
Merge pull request #1821 from FengYang/info

virt.qemu_storage: Always check the image by qemu-img info, if glusterfs...