Add PVHVM multiprocessor support:
We need the hypervisor to be set up before cpus attaches.
Move hypervisor setup to a new function xen_hvm_init(), called at the
beggining of mainbus_attach(). This function searches the cfdata[] array
to see if the hypervisor device is enabled (so you can disable PV
support with
disable hypervisor
from userconf).
For HVM, ci_cpuid doens't match the virtual CPU index needed by Xen.
Introduce ci_vcpuid to cpu_info. Introduce xen_hvm_init_cpu(), to be
called for each CPU in in its context, which initialize ci_vcpuid and
ci_vcpu, and setup the event callback.
Change Xen code to use ci_vcpuid.

Do not call lapic_calibrate_timer() for VM_GUEST_XENPVHVM, we will use
Xen timers.

Don't call lapic_initclocks() from cpu_hatch(); instead set
x86_cpu_initclock_func to lapic_initclocks() in lapic_calibrate_timer(),
and call *(x86_cpu_initclock_func)() from cpu_hatch().
Also call x86_cpu_initclock_func from cpu_attach() for the boot CPU.
As x86_cpu_initclock_func is called for all CPUs, x86_initclock_func can
be a NOP for lapic timer.

Reorganize Xen code for x86_initclock_func/x86_cpu_initclock_func.
Move x86_cpu_idle_xen() to hypervisor_machdep.c

(bouyer)