x86emul: support clzero

x86emul: support clzero

... in anticipation of this possibly going to get used by guests for
basic thinks like memset() or clearing or pages.

Since the emulation doesn't use clzero itself, checking the guest's
CPUID for the feature to be exposed is (intentionally) being avoided
here. All that's required is sensible guest side data for the clflush
line size.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Aravind Gopalakrishnan <aravind.gopalakrishnan@amd.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>