virsh: Fix memory leak in cmdNetworkDHCPLeases
    
    After cidr_format is allocated by virAsprintf and used by vshPrintExtra
    it needs to be freed.
    
    Fix the following memory leak from valgrind:
     18 bytes in 1 blocks are definitely lost in loss record 41 of 192
        at 0x4C29BBD: malloc (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
        by 0x85CE36F: __vasprintf_chk (vasprintf_chk.c:80)
        by 0x4EE52D5: UnknownInlinedFun (stdio2.h:210)
        by 0x4EE52D5: virVasprintfInternal (virstring.c:459)
        by 0x4EE53CA: virAsprintfInternal (virstring.c:480)
        by 0x14FE96: cmdNetworkDHCPLeases (virsh-network.c:1378)
        by 0x13006B: vshCommandRun (virsh.c:1915)
        by 0x12A9E1: main (virsh.c:3699)
    
    Signed-off-by: Luyao Huang <lhuang@redhat.com>