use virBitmapFree instead of VIR_FREE for cpumask

Found by 'git grep FREE.*cpumask' after looking at 31f1f6bf.

src/conf/domain_conf.c

@@ -13542,7 +13542,7 @@ virDomainVcpuPinDel(virDomainDefPtr def, int vcpu)
 
     for (n = 0; n < def->cputune.nvcpupin; n++) {
         if (vcpupin_list[n]->vcpuid == vcpu) {
-            VIR_FREE(vcpupin_list[n]->cpumask);
+            virBitmapFree(vcpupin_list[n]->cpumask);
             VIR_FREE(vcpupin_list[n]);
             memmove(&vcpupin_list[n],
                     &vcpupin_list[n+1],

src/libxl/libxl_driver.c

@@ -651,7 +651,7 @@ libxlVmCleanup(libxlDriverPrivatePtr driver,
     /* Remove any cputune settings */
     if (vm->def->cputune.nvcpupin) {
         for (i = 0; i < vm->def->cputune.nvcpupin; ++i) {
-            VIR_FREE(vm->def->cputune.vcpupin[i]->cpumask);
+            virBitmapFree(vm->def->cputune.vcpupin[i]->cpumask);
             VIR_FREE(vm->def->cputune.vcpupin[i]);
         }
         VIR_FREE(vm->def->cputune.vcpupin);