better handle table _IMAGE_OS_TABLE

[jscotka]

Hi,
I found issue, that I have own virt-builder repo with teplates.
Iti s okay in case I have there fedora-21 and similar,
But in case I have there distro like
fedora-22 or another name for fedora-21 like: fedora21-cockpit
it leads to problem with code:
'--os-variant', _get_image_os(template),

Now I patched that table et the begin of file, but better solution is to use some mapping to some older one:
like if you don't know that (it means not in table and also libvirt does not know that) map it to for example fedora20
or another possibility is to add option to force use some version for --os-variant
Regards
Honza