kvm: x86: Add support for fast CR3 switch across different MMU modes

This generalizes the lockless CR3 switch path to be able to work
across different MMU modes (e.g. nested vs non-nested) by checking
that the expected page role of the new root page matches the page role
of the previously stored root page in addition to checking that the new
CR3 matches the previous CR3. Furthermore, instead of loading the
hardware CR3 in fast_cr3_switch(), it is now done in vcpu_enter_guest(),
as by that time the MMU context would be up-to-date with the VCPU mode.

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