KVM: x86: Add CONFIG_KVM_IOAPIC to allow disabling in-kernel I/O APIC

KVM: x86: Add CONFIG_KVM_IOAPIC to allow disabling in-kernel I/O APIC

Add a Kconfig to allow building KVM without support for emulating a I/O
APIC, PIC, and PIT, which is desirable for deployments that effectively
don't support a fully in-kernel IRQ chip, i.e. never expect any VMM to
create an in-kernel I/O APIC.  E.g. compiling out support eliminates a few
thousand lines of guest-facing code and gives security folks warm fuzzies.

As a bonus, wrapping relevant paths with CONFIG_KVM_IOAPIC #ifdefs makes
it much easier for readers to understand which bits and pieces exist
specifically for fully in-kernel IRQ chips.

Opportunistically convert all two in-kernel uses of __KVM_HAVE_IOAPIC to
CONFIG_KVM_IOAPIC, e.g. rather than add a second #ifdef to generate a stub
for kvm_arch_post_irq_routing_update().

Acked-by: Kai Huang <kai.huang@intel.com>
Link: https://lore.kernel.org/r/20250611213557.294358-15-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>