KVM: x86: Explicitly check for in-kernel PIC when getting ExtINT

Explicitly check for an in-kernel PIC when checking for a pending ExtINT
in the PIC.  Effectively swapping the split vs. full irqchip logic will
allow guarding the in-kernel I/O APIC (and PIC) emulation with a Kconfig,
and also makes it more obvious that kvm_pic_read_irq() won't result in a
NULL pointer dereference.

Opportunistically add WARNs in the fallthrough path, mostly to document
that the userspace ExtINT logic is only relevant to split IRQ chips.

Acked-by: Kai Huang <kai.huang@intel.com>
Link: https://lore.kernel.org/r/20250611213557.294358-13-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

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