framebuffer.c expects DIRTY_MEMORY_VGA logging to be always on, but that
will not be the case soon.  Because framebuffer.c computes the memory
region on the fly for every update (with memory_region_find), it cannot
enable/disable logging by itself.

Instead, always treat updates as invalidations if dirty logging is
not enabled, assuming that the board will enable logging on the
RAM region that includes the framebuffer.

Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>