qemu_cpu: SKIP tests not supported by qemu bin under test

For now, verify whether QEMU has the capabilities we
want when executing tests meant for a specific version.

For example, in the RHEL6 related variants, verify
whether qemu under test is a RHEL6 version, and
for QEMU 1.3 variants, verify whether the qemu version
is > 1.3.0. We might generalize this check in future
patches, moving it to pre/postprocessing code.

Also, modify the exception handling in the test
code, and let exceptions go through the main test
scope unmodified. Previously, TestNAErrors were
being transformed in TestFails, defeating the
purpose of using them in the first place.

CC: Eduardo Habkost <ehabkost@redhat.com>
CC: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
qemu_cpu: Add qemu13 and rhel64 variants to default.vendor.nokvm

Both the AMD and Intel variants are not appropriate to
run on qemu versions < 1.3.0 or not RHEL6 anyway. This
way, we don't have a bogus failure when executing the
test on Fedora 18's qemu-kvm, for example, only SKIPs.

CC: Eduardo Habkost <ehabkost@redhat.com>
CC: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>