KVM: x86: Move kvm_intr_is_single_vcpu() to lapic.c

KVM: x86: Move kvm_intr_is_single_vcpu() to lapic.c

Move kvm_intr_is_single_vcpu() to lapic.c, drop its export, and make its
"fast" helper local to lapic.c.  kvm_intr_is_single_vcpu() is only usable
if the local APIC is in-kernel, i.e. it most definitely belongs in the
local APIC code.

No functional change intended.

Fixes: cf04ec393ed0 ("KVM: x86: Dedup AVIC vs. PI code for identifying target vCPU")
Link: https://lore.kernel.org/r/20250919003303.1355064-4-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>