qemu: Error out if the bitmap for pinning is all clear

qemu: Error out if the bitmap for pinning is all clear

For both "live" and "config" changes of vcpupin and emulatorpin, an
all clear bitmap doesn't make sense, and it can just cause corruptions.
E.g (similar for emulatorpin).

% virsh vcpupin hame 0 8,^8 --config

% virsh vcpupin hame
VCPU: CPU Affinity
----------------------------------
   0:
   1: 0-63
   2: 0-63
   3: 0-63

% virsh dumpxml hame | grep cpuset
    <vcpupin vcpu='0' cpuset=''/>

% virsh start hame
error: Failed to start domain hame
error: An error occurred, but the cause is unknown