i8254: Factor out base class for KVM reuse

Applying the concept used for the *PICs once again: establish a base
class for the i8254 that can be used both by the current user space
emulation and the upcoming KVM in-kernel version. We share most of the
public interface of the i8254, specifically to the pcspk, vmstate, reset
and certain init parts.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Avi Kivity <avi@redhat.com>