KVM: x86: keep track of LVT0 changes under APICv

Memory-mapped LVT0 register already contains the new value when APICv
traps so we can't directly detect a change.
Memorize a bit we are interested in to enable legacy NMI watchdog.

Suggested-by: Yoshida Nobuo <yoshida.nb@ncos.nec.co.jp>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>