kvm: x86: Support selectively freeing either current or previous MMU root

kvm_mmu_free_roots() now takes a mask specifying which roots to free, so
that either one of the roots (active/previous) can be individually freed
when needed.

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