x86/cpu: Introduce x86_cpuid_vendor_to_str() and drop cpu_dev.c_vendor[]

x86/cpu: Introduce x86_cpuid_vendor_to_str() and drop cpu_dev.c_vendor[]

cpu_dev.c_vendor[] is a char[8] array which is printed using %s in two
locations.  This leads to subtle lack-of-NUL bugs when using an 8 character
vendor name.

Introduce x86_cpuid_vendor_to_str() to turn an x86_vendor into a printable
string, use it in the two locations that c_vendor is used, and drop c_vendor.

This drops the final user of X86_VENDOR_NUM, so drop that as well.

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