Now that memory_region_sync_dirty_bitmap is NULL, we can unify its
loop with memory_global_dirty_log_sync's.  The only difference is
that memory_region_sync_dirty_bitmap will no longer call log_sync on
FlatRanges that do have a zero dirty_log_mask, but this is okay because
video memory is always registered with the dirty page logging mechanism.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>