serial_hd(i) is NULL if and only if i >= serial_max_hds(). Test
serial_hd(i) instead of bounding the loop at serial_max_hds(),
thus removing one more function that vl.c is expected to export.
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>