x86/cpu: Improvements to get_cpu_vendor()

x86/cpu: Improvements to get_cpu_vendor()

Comparing 3 integers is more efficient than using strcmp(), and is more useful
to the gcv_guest case than having to fabricate a suitable string to pass.  The
gcv_host cases have both options easily to hand, and experimentally, the
resulting code is more efficient.

Update the cpu_dev structure to be more efficient.  c_vendor[] only needs to
be 8 bytes long to cover all the CPU drivers Xen has, which avoids storing an
8-byte pointer to 8 bytes of data.  Drop c_ident[1] as we have no CPU drivers
with a second ident string, and turn it into an anonymous union to allow
access to the integer values directly.

This avoids all need for the vendor_id union in update_domain_cpuid_info().

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