xen: fix interrupt routing

xen: fix interrupt routing

Compared to the last version I only added a comment to the code.

- remove i440FX-xen and i440fx_write_config_xen
we don't need to intercept pci config writes to i440FX anymore;

- introduce PIIX3-xen and piix3_write_config_xen
we do need to intercept pci config write to the PCI-ISA bridge to update
the PCI link routing;

- set the number of PIIX3-xen interrupts line to 128;

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Alexander Graf <agraf@suse.de>