KVM: x86: Move kvm_{request,free}_irq_source_id() to i8254.c (PIT)

KVM: x86: Move kvm_{request,free}_irq_source_id() to i8254.c (PIT)

Move kvm_{request,free}_irq_source_id() to i8254.c, i.e. the dedicated PIT
emulation file, in anticipation of removing them entirely in favor of
hardcoding the PIT's "requested" source ID (the source ID can only ever be
'2', and the request can never fail).

No functional change intended.

Acked-by: Kai Huang <kai.huang@intel.com>
Link: https://lore.kernel.org/r/20250611213557.294358-10-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>