virtual_disks_multivms: Add test case for shared readonly iso img

Add test case for readonly iso for multiple vms, also add check
with seclabel of shared img and iso.

Signed-off-by: Wayne Sun <gsun@redhat.com>
Merge pull request #316 from waynesun09/svirt_multivms_ro

virtual_disks_multivms: Add test case for shared readonly iso img