x86/HVM: consolidate and sanitize CR4 guest reserved bit determination

x86/HVM: consolidate and sanitize CR4 guest reserved bit determination

First of all, this is needed by just a single source file, so it gets
moved there instead of getting fed to the compiler for most other
source files too. With that it becomes sensible for this to no longer
be a macro, allowing elimination of the mostly redundant helpers
hvm_vcpu_has_{smep,smap}(). And finally, following the model SMEP and
SMAP already used, tie the determination of reserved bits to the
features the guest is shown rather than the host's.

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