fix memleak in qemuRestoreCgroupState

fix memleak in qemuRestoreCgroupState

 131,088 bytes in 16 blocks are definitely lost in loss record 2,174 of 2,176
    at 0x4C29BFD: malloc (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
    by 0x4C2BACB: realloc (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
    by 0x52A026F: virReallocN (viralloc.c:245)
    by 0x52BFCB5: saferead_lim (virfile.c:1268)
    by 0x52C00EF: virFileReadLimFD (virfile.c:1328)
    by 0x52C019A: virFileReadAll (virfile.c:1351)
    by 0x52A5D4F: virCgroupGetValueStr (vircgroup.c:763)
    by 0x1DDA0DA3: qemuRestoreCgroupState (qemu_cgroup.c:805)
    by 0x1DDA0DA3: qemuConnectCgroup (qemu_cgroup.c:857)
    by 0x1DDB7BA1: qemuProcessReconnect (qemu_process.c:3694)
    by 0x52FD171: virThreadHelper (virthread.c:206)
    by 0x82B8DF4: start_thread (pthread_create.c:308)
    by 0x85C31AC: clone (clone.S:113)

Signed-off-by: Luyao Huang <lhuang@redhat.com>