KVM: x86/mmu: Do not set paging-related bits in MMU role if CR0.PG=0

Don't set CR0/CR4/EFER bits in the MMU role if paging is disabled, paging
modifiers are irrelevant if there is no paging in the first place.
Somewhat arbitrarily clear gpte_is_8_bytes for shadow paging if paging is
disabled in the guest.  Again, there are no guest PTEs to process, so the
size is meaningless.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210622175739.3610207-27-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>