qemuAgentDispose: Reset lastError
qemuAgentDispose: Reset lastError

When an error occurred in qemuAgentIO, it will be saved in mon->lastError,
but it will not be freed at the end.  Present since commit c160ce33;
and compare to commit 9cc8a5af fixing the same problem in qemu_monitor.c.

==22219== 54 bytes in 1 blocks are definitely lost in loss record 982 of 1,379
==22219==    at 0x4C26B9B: malloc (vg_replace_malloc.c:263)
==22219==    by 0x8520521: strdup (in /lib64/libc-2.11.3.so)
==22219==    by 0x52E99CB: virStrdup (virstring.c:554)
==22219==    by 0x52B44C4: virCopyError (virerror.c:195)
==22219==    by 0x52B5123: virCopyLastError (virerror.c:312)
==22219==    by 0x10905877: qemuAgentIO (qemu_agent.c:660)
==22219==    by 0x52B6122: virEventPollDispatchHandles (vireventpoll.c:501)
==22219==    by 0x52B7AEA: virEventPollRunOnce (vireventpoll.c:647)
==22219==    by 0x52B5C1B: virEventRunDefaultImpl (virevent.c:274)
==22219==    by 0x54181FD: virNetServerRun (virnetserver.c:1112)
==22219==    by 0x11EF4D: main (libvirtd.c:1513)

Signed-off-by: Zhou Yimin <301f1aa1a5d5742a74cf62f513901bacca83bb66@huawei.com>
Signed-off-by: Eric Blake <a9da18479c871fe903d9590822cfe2629c6a4d11@redhat.com>

jfehlig/libvirt,VenkatDatta/libvirt,crobinso/libvirt,nertpinx/libvirt,crobinso/libvirt,jardasgit/libvirt,zippy2/libvirt,crobinso/libvirt,jardasgit/libvirt,rlaager/libvirt,eskultety/libvirt,andreabolognani/libvirt,elmarco/libvirt,elmarco/libvirt,agx/libvirt,andreabolognani/libvirt,jardasgit/libvirt,datto/libvirt,fabianfreyer/libvirt,jfehlig/libvirt,datto/libvirt,agx/libvirt,olafhering/libvirt,olafhering/libvirt,zippy2/libvirt,libvirt/libvirt,rlaager/libvirt,elmarco/libvirt,eskultety/libvirt,olafhering/libvirt,jfehlig/libvirt,fabianfreyer/libvirt,nertpinx/libvirt,andreabolognani/libvirt,fabianfreyer/libvirt,zippy2/libvirt,elmarco/libvirt,andreabolognani/libvirt,rlaager/libvirt,shugaoye/libvirt,shugaoye/libvirt,andreabolognani/libvirt,libvirt/libvirt,olafhering/libvirt,shugaoye/libvirt,fabianfreyer/libvirt,datto/libvirt,libvirt/libvirt,rlaager/libvirt,zippy2/libvirt,datto/libvirt,agx/libvirt,nertpinx/libvirt,VenkatDatta/libvirt,VenkatDatta/libvirt,jardasgit/libvirt,nertpinx/libvirt,jardasgit/libvirt,nertpinx/libvirt,taget/libvirt,shugaoye/libvirt,eskultety/libvirt,eskultety/libvirt,jfehlig/libvirt,fabianfreyer/libvirt,agx/libvirt,shugaoye/libvirt,agx/libvirt,crobinso/libvirt,taget/libvirt,datto/libvirt,rlaager/libvirt,elmarco/libvirt,taget/libvirt,eskultety/libvirt,libvirt/libvirt,VenkatDatta/libvirt,taget/libvirt,VenkatDatta/libvirt,taget/libvirt