memory: Fix incorrect calls of log_global_start/stop

memory: Fix incorrect calls of log_global_start/stop

We should only call the log_global_start/stop when the global dirty track
bitmask changes from zero<->non-zero.

No real issue reported for this yet probably because no immediate user to
enable both dirty rate measurement and migration at the same time.  However
it'll be good to be prepared for it.

Fixes: 63b41db4bc ("memory: make global_dirty_tracking a bitmask")
Cc: qemu-stable@nongnu.org
Cc: Hyman Huang <huangy81@chinatelecom.cn>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Dr. David Alan Gilbert <dgilbert@redhat.com>
Cc: Juan Quintela <quintela@redhat.com>
Cc: David Hildenbrand <david@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Message-Id: <20211130080028.6474-1-peterx@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>