xen: Allow platform PCI interrupt to be shared

When we don't use the per-CPU vector callback, we ask Xen to deliver event
channel interrupts as INTx on the PCI platform device. As such, it can be
shared with INTx on other PCI devices.

Set IRQF_SHARED, and make it return IRQ_HANDLED or IRQ_NONE according to
whether the evtchn_upcall_pending flag was actually set. Now I can share
the interrupt:

 11:         82          0   IO-APIC  11-fasteoi   xen-platform-pci, ens4

Drop the IRQF_TRIGGER_RISING. It has no effect when the IRQ is shared,
and besides, the only effect it was having even beforehand was to trigger
a debug message in both I/OAPIC and legacy PIC cases:

[    0.915441] genirq: No set_type function for IRQ 11 (IO-APIC)
[    0.951939] genirq: No set_type function for IRQ 11 (XT-PIC)

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Reviewed-by: Juergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/r/f9a29a68d05668a3636dd09acd94d970269eaec6.camel@infradead.org
Signed-off-by: Juergen Gross <jgross@suse.com>