KVM I/O infrastructure

This introduces an "Ioport" interface and wires KVM_EXIT_IO hypervisor
trap to delegate I/O requests to an underlying device.

Signed-off-by: Pekka Enberg <penberg@iki.fi>
Real-time clock (RTC)

This adds initial RTC support. This just enough to make SeaBIOS limp
along and access the next unsupported ioport.

Signed-off-by: Pekka Enberg <penberg@iki.fi>