Let build_header() support specified OEM table id so that we can build
multiple SSDT later

If the oem table id is not specified (aka, NULL), we use the default id
instead as the previous behavior
Reviewed-by: NStefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: NXiao Guangrong <guangrong.xiao@linux.intel.com>
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>