qemuMonitorDispose: Reset lastError
    
    Since the 90139a62 commit the error is copied into mon->lastError but
    it's never freed from there.
    
    ==31989== 395 bytes in 1 blocks are definitely lost in loss record 877 of 978
    ==31989==    at 0x4A06C2B: malloc (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
    ==31989==    by 0x7EAF129: strdup (in /lib64/libc-2.15.so)
    ==31989==    by 0x50D586C: virStrdup (virstring.c:554)
    ==31989==    by 0x50976C1: virCopyError (virerror.c:191)
    ==31989==    by 0x5097A35: virCopyLastError (virerror.c:312)
    ==31989==    by 0x114909A9: qemuMonitorIO (qemu_monitor.c:690)
    ==31989==    by 0x509BEDE: virEventPollDispatchHandles (vireventpoll.c:501)
    ==31989==    by 0x509C701: virEventPollRunOnce (vireventpoll.c:648)
    ==31989==    by 0x509A620: virEventRunDefaultImpl (virevent.c:274)
    ==31989==    by 0x520D21C: virNetServerRun (virnetserver.c:1112)
    ==31989==    by 0x11F368: main (libvirtd.c:1513)
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>