qemu: cache host arch separately from virCapsPtr

qemu: cache host arch separately from virCapsPtr

As part of a goal to eliminate the need to use virCapsPtr for anything
other than the virConnectGetCapabilies() API impl, cache the host arch
against the QEMU driver struct and use that field directly.

In the tests we move virArchFromHost() globally in testutils.c so that
every test runs with a fixed default architecture reported.

Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>