xen: do not initialize the interval timer and PCSPK emulator

PIT and PCSPK are emulated by the hypervisor so we don't need to emulate
them in Qemu: this patch prevents Qemu from waking up needlessly at
PIT_FREQ on Xen.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
