Wrong CPUSET for partial allocation.

Trying to cover the issue https://github.com/Eyescale/Equalizer/issues/167, I was trying to find the cpuset that corresponds to the allocated core. I have used the following allocation command `salloc -N1 -n1 --gres=gpu:3 -p interactive` and I got the following topology map

```
Machine (24GB) cpuset=0x00000001
NUMANode L#0 (P#0 12GB) cpuset=0x00000001
Socket L#0 cpuset=0x00000001
L3 L#0 (12MB) cpuset=0x00000001
L2 L#0 (256KB) cpuset=0x00000001
L1d L#0 (32KB) cpuset=0x00000001
L1i L#0 (32KB) cpuset=0x00000001
Core L#0 cpuset=0x00000001
PU L#0 (P#0) cpuset=0x00000001
NUMANode L#1 (P#1 12GB) cpuset=0x0
HostBridge L#0
PCIBridge
PCI 10de:1080
PCIBridge
PCI 10de:1080
PCIBridge
PCI 8086:10d3
Net L#0 "eth0"
PCIBridge
PCI 8086:10d3
Net L#1 "eth1"
PCIBridge
PCI 102b:0532
PCI 8086:3a22
Block L#2 "sda"
HostBridge L#6
PCIBridge
PCI 8086:10fb
Net L#3 "eth2"
PCI 8086:10fb
Net L#4 "eth3"
PCIBridge
PCI 1077:7322
Net L#5 "ib0"
OpenFabrics L#6 "qib0"
PCIBridge
PCI 10de:1080
```

If I try to get the cpuset of the node connected to the host bridges of the allocated GPUs, I get a null cpuset for the third GPU, but I get for the second CPU the wrong one 0x0 which should be according to the previous figure 0x00000001().
The way I am getting the cpu sets is by
1. Checking the corresponding GPU to the attached screen

```
if (HWLOC_OBJ_OSDEV_GPU == osdev->attr->osdev.type
&& osdev->name
&& sscanf(osdev->name, ":%u.%u", &x, &y) == 2
&& port == x && device == y)
```

This gives me the osdev.
2. I get the parent of the osdev which should be the PCI device.

```
hwloc_obj_t parent = osdev->parent;
```
1. Getting the host bridge by the PCI bus

```
hwloc_obj_t host_bridge;
host_bridge = hwloc_get_hostbridge_by_pcibus (topology,
parent->attr->pcidev.domain,
parent->attr->pcidev.bus);
```
1. Getting the cpuset of the previous sibling of the host bridge, which is the NUMANode which should have the same cpuset of the undelying socket.

```
hwloc_cpuset_t cpuset = hwloc_bitmap_dup(host_bridge->prev_sibling->cpuset);

```