cpuid: Detect missing CPU model

The behavior of qemu_vm when a CPU model is missing has changed. This
requires our test code to detect QEMU errors when a CPU model is
missing.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Merge pull request #1371 from ehabkost/pull-requests/detect-missing-cpu-model-2014-01-20

cpuid: Detect missing CPU model