When CPU comparison APIs return VIR_CPU_COMPARE_INCOMPATIBLE, the caller
has no clue why the CPU is considered incompatible with host CPU. And in
some cases, it would be nice to be able to get such info in a client
rather than having to look in logs.

To achieve this, the APIs can be told to return VIR_ERR_CPU_INCOMPATIBLE
error for incompatible CPUs and the reason will be described in the
associated error message.
Signed-off-by: NJiri Denemark <jdenemar@redhat.com>