i8254: Factor out pit_get_channel_info

i8254: Factor out pit_get_channel_info

Instead of providing 4 individual query functions for mode, gate, output
and initial counter state, introduce a service that queries all
information at once. This comes with tiny additional costs for
pcspk_callback but with a much cleaner interface. Also, it will simplify
the implementation of the KVM in-kernel PIT model.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>