kvm: x86: Add multi-entry LRU cache for previous CR3s

Adds support for storing multiple previous CR3/root_hpa pairs maintained
as an LRU cache, so that the lockless CR3 switch path can be used when
switching back to any of them.

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