rpc: fix keep alive timer segfault

rpc: fix keep alive timer segfault

ka maybe have been freeed in virObjectUnref, application using
virKeepAliveTimer will segfault when unlock ka. We should keep
ka's refs positive before using it.

#0  0x00007fd8f79970e8 in virClassIsDerivedFrom (klass=0xdeadbeef, parent=0x7fd8e8001b80) at util/virobject.c:169
#1  0x00007fd8f799742e in virObjectIsClass (anyobj=anyobj entry=0x7fd8e800b9c0, klass=＜optimized out＞) at util/virobject.c:365
#2  0x00007fd8f79974e4 in virObjectUnlock (anyobj=0x7fd8e800b9c0) at util/virobject.c:338
#3  0x00007fd8f7ac477e in virKeepAliveTimer (timer=＜optimized out＞, opaque=0x7fd8e800b9c0) at rpc/virkeepalive.c:177
#4  0x00007fd8f7e5c9cf in libvirt_virEventInvokeTimeoutCallback () from /usr/lib64/python2.7/site-packages/libvirtmod.so
#5  0x00007fd8ff64db94 in PyEval_EvalFrameEx () from /lib64/libpython2.7.so.1.0
#6  0x00007fd8ff64f1ad in PyEval_EvalCodeEx () from /lib64/libpython2.7.so.1.0
#7  0x00007fd8ff64d85f in PyEval_EvalFrameEx () from /lib64/libpython2.7.so.1.0
#8  0x00007fd8ff64d950 in PyEval_EvalFrameEx () from /lib64/libpython2.7.so.1.0
#9  0x00007fd8ff64d950 in PyEval_EvalFrameEx () from /lib64/libpython2.7.so.1.0
#10 0x00007fd8ff64f1ad in PyEval_EvalCodeEx () from /lib64/libpython2.7.so.1.0
#11 0x00007fd8ff5dc098 in function_call () from /lib64/libpython2.7.so.1.0
#12 0x00007fd8ff5b7073 in PyObject_Call () from /lib64/libpython2.7.so.1.0
#13 0x00007fd8ff5c6085 in instancemethod_call () from /lib64/libpython2.7.so.1.0
#14 0x00007fd8ff5b7073 in PyObject_Call () from /lib64/libpython2.7.so.1.0
#15 0x00007fd8ff648ff7 in PyEval_CallObjectWithKeywords () from /lib64/libpython2.7.so.1.0
#16 0x00007fd8ff67d7e2 in t_bootstrap () from /lib64/libpython2.7.so.1.0
#17 0x00007fd8ff358df3 in start_thread () from /lib64/libpthread.so.0
#18 0x00007fd8fe97d3ed in clone () from /lib64/libc.so.6

Signed-off-by: Yi Wang <wang.yi59@zte.com.cn>
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>