serial: fix multi-pci card error cleanup.

Put the number of serial ports into a local variable in
multi_serial_pci_realize, then increment the port count
(pci->ports) as we initialize the serial port cores.

Now pci->ports always holds the number of successfully
initialized ports and we can use multi_serial_pci_exit
to properly cleanup the already initialized bits in case
of a init failure.

https://bugzilla.redhat.com/show_bug.cgi?id=970551

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
