qemu: Only probe available machine types

qemu: Only probe available machine types

Since we know the full list of machine types supported
by the QEMU binary when probing machine type properties,
we can save some work (and eventually test suite churn,
as more architecture-specific machine types need to be
probed) by only probing machines that we know exist.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Pavel Hrdina <phrdina@redhat.com>