virtio-serial: Check if more max_ports specified than we can handle

Currently virtio-serial supports a maximum of 31 ports. Specifying the
'max_ports' parameter to be > 31 on the cmd line causes badness.

Ensure we initialise virtio-serial only if max_ports is within the
supported range.

Signed-off-by: Amit Shah <amit.shah@redhat.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
