The setting up of such vectors really is a boot-time-only thing. vPMU also
doesn't need any vector when it's no in use anyway (which is the default).

1: guest: move allocation of Xen upcall vector to init code
2: MCE: restrict allocation of thermal and CMCI vector to BSP
3: IRQ: direct-APIC-vector setting is now init-only
4: vPMU: don't statically reserve the interrupt vector

Jan