tests: Add support for multiple variants of data for the same qemu version+architecture

tests: Add support for multiple variants of data for the same qemu version+architecture

'qemucapabilitiestest' and other users of the capability data can
benefit from adding a discriminator string to have multiple instances
for the same version+architecture tuple.

This will in the future allow us to have specific capability versions
for test cases which require a specific host feature or are based on a
different operating system.

Add the basic skeleton for parsing the variant string and passing it
around into test cases.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Martin Kletzander <mkletzan@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>