qemuSetupCgroupForVcpu: Fix memleak
    
    ==19015== 1,064 (656 direct, 408 indirect) bytes in 2 blocks are definitely lost in loss record 1,002 of 1,049
    ==19015==    at 0x4C2C070: calloc (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
    ==19015==    by 0x52AD74B: virAlloc (viralloc.c:144)
    ==19015==    by 0x52B47CA: virCgroupNew (vircgroup.c:1057)
    ==19015==    by 0x52B53E5: virCgroupNewVcpu (vircgroup.c:1451)
    ==19015==    by 0x1DD85A40: qemuSetupCgroupForVcpu (qemu_cgroup.c:1013)
    ==19015==    by 0x1DDA66EA: qemuProcessStart (qemu_process.c:4844)
    ==19015==    by 0x1DDF1807: qemuDomainObjStart (qemu_driver.c:7265)
    ==19015==    by 0x1DDF1A66: qemuDomainCreateWithFlags (qemu_driver.c:7320)
    ==19015==    by 0x1DDF1ACD: qemuDomainCreate (qemu_driver.c:7337)
    ==19015==    by 0x53F87EA: virDomainCreate (libvirt-domain.c:6820)
    ==19015==    by 0x12690A: remoteDispatchDomainCreate (remote_dispatch.h:3481)
    ==19015==    by 0x126827: remoteDispatchDomainCreateHelper (remote_dispatch.h:3457)
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>