Add ci_cpuid_level and ci_vendor holding the per-CPU basic cpuid

Add ci_cpuid_level and ci_vendor holding the per-CPU basic cpuid
level and a numeric mapping of the cpu vendor, both from CPUID(0).
Convert the general use of strcmp(cpu_vendor) to simple numeric
tests of ci_vendor.  Track the minimum of all ci_cpuid_level in the
cpuid_level global and continue to use that for what we vmm exposes.

AMD testing help matthieu@ krw@
ok miod@ deraadt@ cheloha@