hw/intc: Generalize APIC helper names from kvm_* to accel_*

hw/intc: Generalize APIC helper names from kvm_* to accel_*

Rename APIC helper functions to use an accel_* prefix instead of kvm_*
to support use by accelerators other than KVM. This is a preparatory
step for integrating MSHV support with common APIC logic.

Signed-off-by: Magnus Kulke <magnuskulke@linux.microsoft.com>
Link: https://lore.kernel.org/r/20250916164847.77883-5-magnuskulke@linux.microsoft.com
[Remove dead definition of mshv_msi_via_irqfd_enabled. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>