admin: Fix memory leak in remoteAdminConnectClose

admin: Fix memory leak in remoteAdminConnectClose

When virt-admin is run with valgrind, this kind of output can be obtained:

HEAP SUMMARY:
  in use at exit: 134,589 bytes in 1,031 blocks
  total heap usage: 2,667 allocs, 1,636 frees, 496,755 bytes allocated

88 bytes in 1 blocks are definitely lost in loss record 82 of 128
 at 0x4C2A9C7: calloc (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
 by 0x52F6D1F: virAllocVar (viralloc.c:560)
 by 0x5350268: virObjectNew (virobject.c:193)
 by 0x53503E0: virObjectLockableNew (virobject.c:219)
 by 0x4E3BBCB: virAdmConnectNew (datatypes.c:832)
 by 0x4E38495: virAdmConnectOpen (libvirt-admin.c:209)
 by 0x10C541: vshAdmConnect (virt-admin.c:107)
 by 0x10C7B2: vshAdmReconnect (virt-admin.c:163)
 by 0x10CC7C: cmdConnect (virt-admin.c:298)
 by 0x110838: vshCommandRun (vsh.c:1224)
 by 0x10DFD8: main (virt-admin.c:862)

 LEAK SUMMARY:
    definitely lost: 88 bytes in 1 blocks
    indirectly lost: 0 bytes in 0 blocks
    possibly lost: 0 bytes in 0 blocks
    still reachable: 134,501 bytes in 1,030 blocks
    suppressed: 0 bytes in 0 blocks

This is because virNetClientSetCloseCallback was being reinitialized
incorrectly. By resetting the callbacks in a proper way, the leak is fixed.