KVM: x86: Add & use kvm_vcpu_is_legal_cr3() to check CR3's legality

Add and use kvm_vcpu_is_legal_cr3() to check CR3's legality to provide
a clear distinction between CR3 and GPA checks.  This will allow exempting
bits from kvm_vcpu_is_legal_cr3() without affecting general GPA checks,
e.g. for upcoming features that will use high bits in CR3 for feature
enabling.

No functional change intended.

Signed-off-by: Binbin Wu <binbin.wu@linux.intel.com>
Tested-by: Xuelian Guo <xuelian.guo@intel.com>
Link: https://lore.kernel.org/r/20230913124227.12574-7-binbin.wu@linux.intel.com


Signed-off-by: Sean Christopherson <seanjc@google.com>