domcaps: Check for architecture more wisely

domcaps: Check for architecture more wisely

https://bugzilla.redhat.com/show_bug.cgi?id=1209948

So we have this bug. The virConnectGetDomainCapabilities() API
performs a couple of checks before it produces any result. One of
the checks is if the architecture requested by user can be run by
the binary (again user provided). However, the check is pretty
dumb. It merely compares if the default binary architecture
matches the one provided by user. However, a qemu binary can run
multiple architectures. For instance: qemu-system-ppc64 can run:
ppc, ppcle, ppc64, ppc64le and ppcemb. The default is ppc64, so
if user requested something else, like ppc64le, the check would
have failed without obvious reason.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>