x86/irq: limit the maximum number of domain PIRQs

x86/irq: limit the maximum number of domain PIRQs

c/s 7e73a6e "have architectures specify the number of PIRQs a hardware domain
gets" increased the default number of pirqs for dom0, as 256 was found to be
too low in some cases.

However, it didn't account for the upper bound presented by the domains EOI
bitmap, registered with the PHYSDEVOP_pirq_eoi_gmfn_v* hypercall.

On a server with 240 cpus, Xen was observed to be attempting to clear the EOI
bit for dom0's pirq 0xb40f, which hit a pagefault.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>