gcccpuopt: handle unspecified QEMU CPU models

QEMU cpuinfo is quite messed up:
  http://lists.gnu.org/archive/html/qemu-devel/2012-05/msg01219.html
though improving:
  http://git.qemu.org/?p=qemu.git;a=commit;h=ffce9ebb
We can detect this anomaly for intel, family 6, model 2 at least.
In that case we'll warn and auto bump to model 6 line as per:
  https://bugzilla.redhat.com/614537