conf: numa: Don't duplicate NUMA cell cpumask

The mask was stored both as a bitmap and as a string. The string is used
for XML output only. Remove the string, as it can be reconstructed from
the bitmap.

The test change is necessary as the bitmap formatter doesn't "optimize"
using the '^' operator.

tests/qemuxml2xmloutdata/qemuxml2xmlout-numatune-memnode.xml

@@ -17,7 +17,7 @@
     <numa>
       <cell id='0' cpus='0' memory='20002' unit='KiB'/>
       <cell id='1' cpus='1-27,29' memory='660066' unit='KiB'/>
-      <cell id='2' cpus='28-31,^29' memory='24002400' unit='KiB'/>
+      <cell id='2' cpus='28,30-31' memory='24002400' unit='KiB'/>
     </numa>
   </cpu>
   <clock offset='utc'/>