xhci: Fix memory leak in xhci_kick_epctx when poweroff GuestOS

xhci: Fix memory leak in xhci_kick_epctx when poweroff GuestOS

start vm with libvirt, when GuestOS running, enter poweroff command using
the xhci keyboard, then ASAN shows memory leak stack：

Direct leak of 80 byte(s) in 5 object(s) allocated from:
    #0 0xfffd1e6431cb in __interceptor_malloc (/lib64/libasan.so.4+0xd31cb)
    #1 0xfffd1e107163 in g_malloc (/lib64/libglib-2.0.so.0+0x57163)
    #2 0xaaad39051367 in qemu_sglist_init /qemu/dma-helpers.c:43
    #3 0xaaad3947c407 in pci_dma_sglist_init /qemu/include/hw/pci/pci.h:842
    #4 0xaaad3947c407 in xhci_xfer_create_sgl /qemu/hw/usb/hcd-xhci.c:1446
    #5 0xaaad3947c407 in xhci_setup_packet /qemu/hw/usb/hcd-xhci.c:1618
    #6 0xaaad3948625f in xhci_submit /qemu/hw/usb/hcd-xhci.c:1827
    #7 0xaaad3948625f in xhci_fire_transfer /qemu/hw/usb/hcd-xhci.c:1839
    #8 0xaaad3948625f in xhci_kick_epctx /qemu/hw/usb/hcd-xhci.c:1991
    #9 0xaaad3948f537 in xhci_doorbell_write /qemu/hw/usb/hcd-xhci.c:3158
    #10 0xaaad38bcbfc7 in memory_region_write_accessor /qemu/memory.c:483
    #11 0xaaad38bc654f in access_with_adjusted_size /qemu/memory.c:544
    #12 0xaaad38bd1877 in memory_region_dispatch_write /qemu/memory.c:1482
    #13 0xaaad38b1c77f in flatview_write_continue /qemu/exec.c:3167
    #14 0xaaad38b1ca83 in flatview_write /qemu/exec.c:3207
    #15 0xaaad38b268db in address_space_write /qemu/exec.c:3297
    #16 0xaaad38bf909b in kvm_cpu_exec /qemu/accel/kvm/kvm-all.c:2383
    #17 0xaaad38bb063f in qemu_kvm_cpu_thread_fn /qemu/cpus.c:1246
    #18 0xaaad39821c93 in qemu_thread_start /qemu/util/qemu-thread-posix.c:519
    #19 0xfffd1c8378bb  (/lib64/libpthread.so.0+0x78bb)
    #20 0xfffd1c77616b  (/lib64/libc.so.6+0xd616b)

Reported-by: Euler Robot <euler.robot@huawei.com>
Signed-off-by: Chen Qun <kuhn.chenqun@huawei.com>
Message-id: 20200110105855.81144-1-kuhn.chenqun@huawei.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>