KVM: x86: Rename irqchip_kernel() to irqchip_full()

KVM: x86: Rename irqchip_kernel() to irqchip_full()

Rename irqchip_kernel() to irqchip_full(), as "kernel" is very ambiguous
due to the existence of split IRQ chip support, where only some of the
"irqchip" is in emulated by the kernel/KVM.  E.g. irqchip_kernel() often
gets confused with irqchip_in_kernel().

Opportunistically hoist the definition up in irq.h so that it's
co-located with other "full" irqchip code in anticipation of wrapping it
all with a Kconfig/#ifdef.

No functional change intended.

Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Acked-by: Kai Huang <kai.huang@intel.com>
Link: https://lore.kernel.org/r/20250611213557.294358-8-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>