KVM: vmx: rename KVM_GUEST_CR0_MASK tp KVM_VM_CR0_ALWAYS_OFF

The KVM_GUEST_CR0_MASK macro tracks CR0 bits that are forced to zero
by the VMX architecture, i.e. CR0.{NW,CD} must always be zero in the
hardware CR0 post-VMXON.  Rename the macro to clarify its purpose,
be consistent with KVM_VM_CR0_ALWAYS_ON and avoid confusion with the
CR0_GUEST_HOST_MASK field.

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Reviewed-by: Jim Mattson <jmattson@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>