KVM: VMX: skip vmx->rmode.vm86_active check on cr0 write if unrestricted guest is enabled

vmx->rmode.vm86_active is never true is unrestricted guest is enabled.
Make it more explicit that neither enter_pmode() nor enter_rmode() is
called in this case.

Signed-off-by: Gleb Natapov <gleb@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>