x86/kvm/mmu.c: add kvm_mmu parameter to kvm_mmu_free_roots()

Add an option to specify which MMU root we want to free. This will
be used when nested and non-nested MMUs for L1 are split.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Sean Christopherson <sean.j.christopherson@intel.com>