libxl: implement virDomainGetVcpuPinInfo

libxl: implement virDomainGetVcpuPinInfo

So that it is possible to query vcpu related information of
a persistent but not running domain, like it is for the QEMU
driver.

In fact, before this patch, we have:
 # virsh list --all
  Id    Name                           State
 ----------------------------------------------------
  5     debian_32                      running
  -     fedora20_64                    shut off
 # virsh vcpuinfo fedora20_64
 error: this function is not supported by the connection driver: virDomainGetVcpuPinInfo

After (same situation as above, i.e., fedora20_64 not running):
 # virsh vcpuinfo fedora20_64
 VCPU:           0
 CPU:            N/A
 State:          N/A
 CPU time        N/A
 CPU Affinity:   yyyyyyyy

 VCPU:           1
 CPU:            N/A
 State:          N/A
 CPU time        N/A
 CPU Affinity:   yyyyyyyy

Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com>
Cc: Jim Fehlig <jfehlig@suse.com>
Cc: Ian Jackson <Ian.Jackson@eu.citrix.com>