Reproduce this bug as follow version:
HOst:
# uname -r
2.6.32-405.el6.x86_64
# rpm -q qemu-kvm
qemu-kvm-0.12.1.2-2.382.el6.x86_64

Guest:
2.6.32-358.el6.x86_64

Steps
1.Boot guest on SRC machine

 /usr/libexec/qemu-kvm -m 4G -smp 4,sockets=1,cores=4,threads=1 -M rhel6.5.0 -enable-kvm -name rhel6.4 -drive file=/mnt/RHEL-Server-6.4-64-virtio.qcow2,if=none,format=qcow2,werror=stop,rerror=stop,media=disk,id=drive-scsi0-disk0 -device virtio-scsi-pci,id=scsi0,addr=0x4 -device scsi-hd,scsi-id=0,lun=0,bus=scsi0.0,drive=drive-scsi0-disk0,id=virtio-disk0 -nodefaults -nodefconfig -monitor stdio -netdev tap,id=bd,script=/etc/qemu-ifup,vhost=on,ifname=qiguo2 -device virtio-net-pci,netdev=bd,mac=54:52:1a:46:1b:03,id=vnic1 -vnc :20 -vga std -boot menu=on -monitor stdio
2.(qemu)set_link bd off

3.migrate -d tcp:10.66.7.82:5999


Results:
wait about 1 min
qemu-kvm: /builddir/build/BUILD/qemu-kvm-0.12.1.2/hw/virtio-net.c:869: virtio_net_save: Assertion `!n->vhost_started' failed.

Program received signal SIGABRT, Aborted.
0x00007ffff5707925 in raise () from /lib64/libc.so.6
Missing separate debuginfos, use: debuginfo-install alsa-lib-1.0.22-3.el6.x86_64 celt051-0.5.1.3-0.el6.x86_64 cyrus-sasl-gssapi-2.1.23-13.el6_3.1.x86_64 cyrus-sasl-lib-2.1.23-13.el6_3.1.x86_64 cyrus-sasl-md5-2.1.23-13.el6_3.1.x86_64 cyrus-sasl-plain-2.1.23-13.el6_3.1.x86_64 db4-4.7.25-17.el6.x86_64 dbus-libs-1.2.24-7.el6_3.x86_64 flac-1.2.1-6.1.el6.x86_64 glib2-2.26.0-3.el6.x86_64 glibc-2.12-1.128.el6.x86_64 gnutls-2.8.5-10.el6_4.2.x86_64 keyutils-libs-1.4-4.el6.x86_64 krb5-libs-1.10.3-10.el6_4.4.x86_64 libICE-1.0.6-1.el6.x86_64 libSM-1.2.1-2.el6.x86_64 libX11-1.5.0-4.el6.x86_64 libXau-1.0.6-4.el6.x86_64 libXext-1.3.1-2.el6.x86_64 libXi-1.6.1-3.el6.x86_64 libXtst-1.2.1-2.el6.x86_64 libaio-0.3.107-10.el6.x86_64 libasyncns-0.8-1.1.el6.x86_64 libcom_err-1.41.12-14.el6_4.2.x86_64 libgcrypt-1.4.5-9.el6_2.2.x86_64 libgpg-error-1.7-4.el6.x86_64 libjpeg-turbo-1.2.1-1.el6.x86_64 libogg-1.1.4-2.1.el6.x86_64 libselinux-2.0.94-5.3.el6_4.1.x86_64 libsndfile-1.0.20-5.el6.x86_64 libtasn1-2.3-3.el6_2.1.x86_64 libuuid-2.17.2-12.13.el6.x86_64 libvorbis-1.2.3-4.el6_2.1.x86_64 libxcb-1.8.1-1.el6.x86_64 nss-softokn-freebl-3.14.3-4.el6.x86_64 openssl-1.0.1e-7.el6.x86_64 pixman-0.26.2-5.el6_4.x86_64 pulseaudio-libs-0.9.21-14.el6_3.x86_64 spice-server-0.12.4-2.el6.x86_64 tcp_wrappers-libs-7.6-57.el6.x86_64 usbredir-0.5.1-1.el6.x86_64 zlib-1.2.3-29.el6.x86_64
(gdb) bt
#0  0x00007ffff5707925 in raise () from /lib64/libc.so.6
#1  0x00007ffff5709105 in abort () from /lib64/libc.so.6
#2  0x00007ffff5700a4e in __assert_fail_base () from /lib64/libc.so.6
#3  0x00007ffff5700b10 in __assert_fail () from /lib64/libc.so.6
#4  0x00007ffff7de6f59 in virtio_net_save (f=0x7ffff9cff7a0, opaque=0x7ffff9cd06c0)
    at /usr/src/debug/qemu-kvm-0.12.1.2/hw/virtio-net.c:869
#5  0x00007ffff7e5b1c0 in vmstate_save (mon=<value optimized out>, f=0x7ffff9cff7a0)
    at /usr/src/debug/qemu-kvm-0.12.1.2/savevm.c:1459
#6  qemu_savevm_state_complete (mon=<value optimized out>, f=0x7ffff9cff7a0) at /usr/src/debug/qemu-kvm-0.12.1.2/savevm.c:1621
#7  0x00007ffff7e51635 in migrate_fd_put_ready (opaque=0x7ffff88fef80) at /usr/src/debug/qemu-kvm-0.12.1.2/migration.c:414
#8  0x00007ffff7e5117f in buffered_put_buffer (opaque=0x7ffff8903070, buf=0x0, pos=0, size=0)
    at /usr/src/debug/qemu-kvm-0.12.1.2/buffered_file.c:165
#9  0x00007ffff7e5158e in migrate_fd_put_notify (opaque=<value optimized out>) at /usr/src/debug/qemu-kvm-0.12.1.2/migration.c:339
#10 0x00007ffff7dd0676 in main_loop_wait (timeout=1000) at /usr/src/debug/qemu-kvm-0.12.1.2/vl.c:4051
#11 0x00007ffff7df314a in kvm_main_loop () at /usr/src/debug/qemu-kvm-0.12.1.2/qemu-kvm.c:2244
#12 0x00007ffff7dd4138 in main_loop (argc=32, argv=<value optimized out>, envp=<value optimized out>)
    at /usr/src/debug/qemu-kvm-0.12.1.2/vl.c:4259
#13 main (argc=32, argv=<value optimized out>, envp=<value optimized out>) at /usr/src/debug/qemu-kvm-0.12.1.2/vl.c:6632
(gdb) 

Verify this bug as follow version:

HOst 
# uname -r 
2.6.32-405.el6.x86_64
# rpm -q qemu-kvm
qemu-kvm-0.12.1.2-2.386.el6.x86_64

Guest:2.6.32-358.el6.x86_64

Steps as same as reproduce

Results: Tried more than 10 times ,not hit any problem .Migrate successfully.

According to above test ,this bug fixed.

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

http://rhn.redhat.com/errata/RHSA-2013-1553.html