x86/cpuid: Drop a guests cached x86 family and model information

x86/cpuid: Drop a guests cached x86 family and model information

The model information isn't used at all, and the family information is only
used once.

Make get_cpu_family() a static inline (as it is just basic calculation, and
the function call is probably more expensive than the function itself) and
rearange the logic to avoid calculating model entirely if the caller doesn't
want it.

Calculate a guests family only when necessary in hvm_select_ioreq_server().

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>