kvm: x86: Add a root_hpa parameter to kvm_mmu->invlpg()

This allows invlpg() to be called using either the active root_hpa
or the prev_root_hpa.

Signed-off-by: Junaid Shahid <junaids@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>