virt: enable detecting QEMU (TCG) via CPUID (#6399)

QEMU >= 2.10 will include a CPUID leaf with value "TCGTCGTCGTCG"
on x86 when running with the TCG CPU emulator:

  https://lists.gnu.org/archive/html/qemu-devel/2017-07/msg05231.html



Existing methods of detecting QEMU are left unchanged for sake of
backcompatibility.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>