KVM: x86: Move kvm_setup_default_irq_routing() into irq.c

KVM: x86: Move kvm_setup_default_irq_routing() into irq.c

Move the default IRQ routing table used for in-kernel I/O APIC and PIC
routing to irq.c, and tweak the name to make it explicitly clear what
routing is being initialized.

In addition to making it more obvious that the so called "default" routing
only applies to an in-kernel I/O APIC, getting it out of irq_comm.c will
allow removing irq_comm.c entirely.  And placing the function alongside
other I/O APIC and PIC code will allow for guarding KVM's in-kernel I/O
APIC and PIC emulation with a Kconfig with minimal #ifdefs.

No functional change intended.

Cc: Kai Huang <kai.huang@intel.com>
Acked-by: Kai Huang <kai.huang@intel.com>
Link: https://lore.kernel.org/r/20250611213557.294358-9-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>