x86/IRQ: allocate guest array of max size only for shareable IRQs

x86/IRQ: allocate guest array of max size only for shareable IRQs

... and increase default "irq-max-guests" to 32.

It's not necessary to have an array of a size more than 1 for non-shareable
IRQs and it might impact scalability in case of high "irq-max-guests"
values being used - every IRQ in the system including MSIs would be
supplied with an array of that size.

Since it's now less impactful to use higher "irq-max-guests" value - bump
the default to 32. That should give more headroom for future systems.

Requested-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Igor Druzhinin <igor.druzhinin@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>