vhost-user-bridge: fix recvmsg iovlen

vhost-user-bridge: fix recvmsg iovlen

After iov_discard_front(), the iov may be smaller than its initial
size. Fixes the heap-buffer-overflow spotted by ASAN:

==9036==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x6060000001e0 at pc 0x7fe632eca3f0 bp 0x7ffddc4a05a0 sp 0x7ffddc49fd48
WRITE of size 32 at 0x6060000001e0 thread T0
    #0 0x7fe632eca3ef  (/lib64/libasan.so.5+0x773ef)
    #1 0x7fe632ecad23 in __interceptor_recvmsg (/lib64/libasan.so.5+0x77d23)
    #2 0x561e7491936b in vubr_backend_recv_cb /home/elmarco/src/qemu/tests/vhost-user-bridge.c:333
    #3 0x561e74917711 in dispatcher_wait /home/elmarco/src/qemu/tests/vhost-user-bridge.c:160
    #4 0x561e7491c3b5 in vubr_run /home/elmarco/src/qemu/tests/vhost-user-bridge.c:725
    #5 0x561e7491c85c in main /home/elmarco/src/qemu/tests/vhost-user-bridge.c:806
    #6 0x7fe631a6c412 in __libc_start_main (/lib64/libc.so.6+0x24412)
    #7 0x561e7491667d in _start (/home/elmarco/src/qemu/build/tests/vhost-user-bridge+0x3967d)

0x6060000001e0 is located 0 bytes to the right of 64-byte region [0x6060000001a0,0x6060000001e0)
allocated by thread T0 here:
    #0 0x7fe632f42848 in __interceptor_malloc (/lib64/libasan.so.5+0xef848)
    #1 0x561e7493acd8 in virtqueue_alloc_element /home/elmarco/src/qemu/contrib/libvhost-user/libvhost-user.c:1848
    #2 0x561e7493c2a8 in vu_queue_pop /home/elmarco/src/qemu/contrib/libvhost-user/libvhost-user.c:1954
    #3 0x561e749189bf in vubr_backend_recv_cb /home/elmarco/src/qemu/tests/vhost-user-bridge.c:297
    #4 0x561e74917711 in dispatcher_wait /home/elmarco/src/qemu/tests/vhost-user-bridge.c:160
    #5 0x561e7491c3b5 in vubr_run /home/elmarco/src/qemu/tests/vhost-user-bridge.c:725
    #6 0x561e7491c85c in main /home/elmarco/src/qemu/tests/vhost-user-bridge.c:806
    #7 0x7fe631a6c412 in __libc_start_main (/lib64/libc.so.6+0x24412)

SUMMARY: AddressSanitizer: heap-buffer-overflow (/lib64/libasan.so.5+0x773ef)
Shadow bytes around the buggy address:
  0x0c0c7fff7fe0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0c0c7fff7ff0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0c0c7fff8000: fa fa fa fa 00 00 00 00 00 00 05 fa fa fa fa fa
  0x0c0c7fff8010: 00 00 00 00 00 00 00 00 fa fa fa fa fd fd fd fd
  0x0c0c7fff8020: fd fd fd fd fa fa fa fa fd fd fd fd fd fd fd fd
=>0x0c0c7fff8030: fa fa fa fa 00 00 00 00 00 00 00 00[fa]fa fa fa
  0x0c0c7fff8040: fd fd fd fd fd fd fd fd fa fa fa fa fd fd fd fd
  0x0c0c7fff8050: fd fd fd fd fa fa fa fa fd fd fd fd fd fd fd fd
  0x0c0c7fff8060: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c0c7fff8070: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c0c7fff8080: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20181109173028.3372-1-marcandre.lureau@redhat.com>
Signed-off-by: Paolo BOnzini <pbonzini@redhat.com>