KVM: x86: Move PIT ioctl helpers to i8254.c

KVM: x86: Move PIT ioctl helpers to i8254.c

Move the PIT ioctl helpers to i8254.c, i.e. to the file that implements
PIT emulation.  Eliminating PIT code in x86.c will allow adding a Kconfig
to control support for in-kernel I/O APIC, PIC, and PIT emulation with
minimal #ifdefs.

Opportunistically make kvm_pit_set_reinject() and kvm_pit_load_count()
local to i8254.c as they were only publicly visible to make them available
to the ioctl helpers.

No functional change intended.

Acked-by: Kai Huang <kai.huang@intel.com>
Link: https://lore.kernel.org/r/20250611213557.294358-6-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>