apic: add global apic_get_class()

apic: add global apic_get_class()

Every configuration has only up to one APIC class and we'll be extending
the class with a function that can be called without an instanced
object, so a direct access to the class is convenient.

This patch will break compilation if some code uses apic_get_class()
with CONFIG_USER_ONLY.

Suggested-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>