hmp: fix migrate status timer leak

hmp: fix migrate status timer leak

Spotted by ASAN doing some manual testing:

Direct leak of 48 byte(s) in 1 object(s) allocated from:
    #0 0x7f5fcdc75e50 in calloc (/lib64/libasan.so.5+0xeee50)
    #1 0x7f5fcd47241d in g_malloc0 (/lib64/libglib-2.0.so.0+0x5241d)
    #2 0x55f989be92ce in timer_new /home/elmarco/src/qq/include/qemu/timer.h:561
    #3 0x55f989be92ff in timer_new_ms /home/elmarco/src/qq/include/qemu/timer.h:630
    #4 0x55f989c0219d in hmp_migrate /home/elmarco/src/qq/hmp.c:2038
    #5 0x55f98955927b in handle_hmp_command /home/elmarco/src/qq/monitor.c:3498
    #6 0x55f98955fb8c in monitor_command_cb /home/elmarco/src/qq/monitor.c:4371
    #7 0x55f98ad40f11 in readline_handle_byte /home/elmarco/src/qq/util/readline.c:393
    #8 0x55f98955fa4f in monitor_read /home/elmarco/src/qq/monitor.c:4354
    #9 0x55f98aae30d7 in qemu_chr_be_write_impl /home/elmarco/src/qq/chardev/char.c:175
    #10 0x55f98aae317a in qemu_chr_be_write /home/elmarco/src/qq/chardev/char.c:187
    #11 0x55f98aae940c in fd_chr_read /home/elmarco/src/qq/chardev/char-fd.c:66
    #12 0x55f98ab63018 in qio_channel_fd_source_dispatch /home/elmarco/src/qq/io/channel-watch.c:84
    #13 0x7f5fcd46c8ac in g_main_dispatch gmain.c:3177

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20180901134652.25884-1-marcandre.lureau@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>