conf: fix memory leak of class_id bitmap

conf: fix memory leak of class_id bitmap

When libvirtd loads active network configs from network state directory,
it should release the class_id memory block which was allocated
at the time of loading xml from network config directory.
virBitmapParse will create a new memory block of bitmap class_id which
causes a memory leak.

This happens when at least one virtual network is active before.

==12234== 8,216 (24 direct, 8,192 indirect) bytes in 1 blocks are definitely \
              lost in loss record 702 of 709
==12234==    at 0x4A06B2F: calloc (vg_replace_malloc.c:593)
==12234==    by 0x37AB04D77D: virAlloc (in /usr/lib64/libvirt.so.0.1000.3)
==12234==    by 0x37AB04EF89: virBitmapNew (in /usr/lib64/libvirt.so.0.1000.3)
==12234==    by 0x37AB0BFB37: virNetworkAssignDef (in /usr/lib64/libvirt.so.0.1000.3)
==12234==    by 0x37AB0BFD31: ??? (in /usr/lib64/libvirt.so.0.1000.3)
==12234==    by 0x37AB0BFE92: virNetworkLoadAllConfigs (in /usr/lib64/libvirt.so.0.1000.3)
==12234==    by 0x10650E5A: ??? (in /usr/lib64/libvirt/connection-driver/libvirt_driver_network.so)
==12234==    by 0x37AB0EB72F: virStateInitialize (in /usr/lib64/libvirt.so.0.1000.3)
==12234==    by 0x40DE04: ??? (in /usr/sbin/libvirtd)
==12234==    by 0x37AB0832E8: ??? (in /usr/lib64/libvirt.so.0.1000.3)
==12234==    by 0x3796807D14: start_thread (in /usr/lib64/libpthread-2.16.so)
==12234==    by 0x37960F246C: clone (in /usr/lib64/libc-2.16.so)