xen/smp: unbind irqworkX when unplugging vCPUs.

The git commit  1ff2b0c3


"xen: implement IRQ_WORK_VECTOR handler" added the functionality
to have a per-cpu "irqworkX" for the IPI APIC functionality.
However it missed the unbind when a vCPU is unplugged resulting
in an orphaned per-cpu interrupt line for unplugged vCPU:

  30:        216          0   xen-dyn-event     hvc_console
  31:        810          4   xen-dyn-event     eth0
  32:         29          0   xen-dyn-event     blkif
- 36:          0          0  xen-percpu-ipi       irqwork2
- 37:        287          0   xen-dyn-event     xenbus
+ 36:        287          0   xen-dyn-event     xenbus
 NMI:          0          0   Non-maskable interrupts
 LOC:          0          0   Local timer interrupts
 SPU:          0          0   Spurious interrupts

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>