KVM: VMX: Fold ept_update_paging_mode_cr0() back into vmx_set_cr0()

Move the CR0/CR3/CR4 shenanigans for EPT without unrestricted guest back
into vmx_set_cr0().  This will allow a future patch to eliminate the
rather gross stuffing of vcpu->arch.cr0 in the paging transition cases
by snapshotting the old CR0.

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210713163324.627647-24-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>