Tie SASL callbacks lifecycle to virNetSessionSASLContext
    
    The array of sasl_callback_t callbacks which is passed to sasl_client_new()
    must be kept alive as long as the created sasl_conn_t object is alive as
    cyrus-sasl uses this structure internally for things like logging, so
    the memory used for callbacks must only be freed after sasl_dispose() has
    been called.
    
    During testing of successful SASL logins with
    virsh -c qemu+tls:///system list --all
    I've been getting invalid read reports from valgrind
    
    ==9237== Invalid read of size 8
    ==9237==    at 0x6E93B6F: _sasl_getcallback (common.c:1745)
    ==9237==    by 0x6E95430: _sasl_log (common.c:1850)
    ==9237==    by 0x16593D87: digestmd5_client_mech_dispose (digestmd5.c:4580)
    ==9237==    by 0x6E91653: client_dispose (client.c:332)
    ==9237==    by 0x6E9476A: sasl_dispose (common.c:851)
    ==9237==    by 0x4E225A1: virNetSASLSessionDispose (virnetsaslcontext.c:678)
    ==9237==    by 0x4CBC551: virObjectUnref (virobject.c:262)
    ==9237==    by 0x4E254D1: virNetSocketDispose (virnetsocket.c:1042)
    ==9237==    by 0x4CBC551: virObjectUnref (virobject.c:262)
    ==9237==    by 0x4E2701C: virNetSocketEventFree (virnetsocket.c:1794)
    ==9237==    by 0x4C965D3: virEventPollCleanupHandles (vireventpoll.c:583)
    ==9237==    by 0x4C96987: virEventPollRunOnce (vireventpoll.c:652)
    ==9237==    by 0x4C94730: virEventRunDefaultImpl (virevent.c:274)
    ==9237==    by 0x12C7BA: vshEventLoop (virsh.c:2407)
    ==9237==    by 0x4CD3D04: virThreadHelper (virthreadpthread.c:161)
    ==9237==    by 0x7DAEF32: start_thread (pthread_create.c:309)
    ==9237==    by 0x8C86EAC: clone (clone.S:111)
    ==9237==  Address 0xe2d61b0 is 0 bytes inside a block of size 168 free'd
    ==9237==    at 0x4A07577: free (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
    ==9237==    by 0x4C73827: virFree (viralloc.c:580)
    ==9237==    by 0x4DE4BC7: remoteAuthSASL (remote_driver.c:4219)
    ==9237==    by 0x4DE33D0: remoteAuthenticate (remote_driver.c:3639)
    ==9237==    by 0x4DDBFAA: doRemoteOpen (remote_driver.c:832)
    ==9237==    by 0x4DDC8DC: remoteConnectOpen (remote_driver.c:1031)
    ==9237==    by 0x4D8595F: do_open (libvirt.c:1239)
    ==9237==    by 0x4D863F3: virConnectOpenAuth (libvirt.c:1481)
    ==9237==    by 0x12762B: vshReconnect (virsh.c:337)
    ==9237==    by 0x12C9B0: vshInit (virsh.c:2470)
    ==9237==    by 0x12E9A5: main (virsh.c:3338)
    
    This commit changes virNetSASLSessionNewClient() to take ownership of the SASL
    callbacks. Then we can free them in virNetSASLSessionDispose() after the corresponding
    sasl_conn_t has been freed.