memory: the only dirty memory flag for users is DIRTY_MEMORY_VGA

DIRTY_MEMORY_MIGRATION is triggered by memory_global_dirty_log_start
and memory_global_dirty_log_stop, so it cannot be used with
memory_region_set_log.

Specify this in the documentation and assert it.

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