apic: Factor out base class for KVM reuse

apic: Factor out base class for KVM reuse

The KVM in-kernel APIC model will reuse parts of the user space model
while providing the same frontend view to guest and most management
interfaces.

Factor out an APIC base class to encapsulate those parts that will be
shared by user space and KVM model. This class offers callback hooks for
init, base/tpr setting, and the external NMI delivery that will be
set via APICCommonInfo structure and implemented specifically in the
subclasses.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>