i386: hvf: implement vga dirty page tracking

This patch implements setting the tracking of dirty vga pages, using hvf's
interface to protect guest memory. It uses the MemoryListener callback
mechanism through .log_start/stop/sync

Signed-off-by: Sergio Andres Gomez Del Real <Sergio.G.DelReal@gmail.com>
Message-Id: <20170913090522.4022-13-Sergio.G.DelReal@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
