qemu: Probe CPU models for KVM and TCG

qemu: Probe CPU models for KVM and TCG

CPU models (and especially some additional details which we will start
probing for later) differ depending on the accelerator. Thus we need to
call query-cpu-definitions in both KVM and TCG mode to get all data we
want.

Tests in tests/domaincapstest.c are temporarily switched to TCG to avoid
having to squash even more stuff into this single patch. They will all
be switched back later in separate commits.

Signed-off-by: Jiri Denemark <jdenemar@redhat.com>