retested on rhel6.7 host version:

2.6.32-573.el6.x86_64
qemu-kvm-0.12.1.2-2.479.el6.x86_64


also had this issue, so this not a regression bug.

retested rhel6.7.Z host version:
2.6.32-573.19.1.el6.x86_64
qemu-kvm-0.12.1.2-2.479.el6_7.3.x86_64

also had this issue.

Moving to 6.9.  Not critical.

Fix included in qemu-kvm-0.12.1.2-2.498.el6

Reproduced this bug in following versions:
Host:
    2.6.32-672.el6.x86_64
    qemu-kvm-0.12.1.2-2.496.el6.x86_64
Guest:
    2.6.32-671.el6.x86_64

Steps to Reproduce:
1.Boot a guest with(e1000/rtl8139):
-device e1000,mac=9a:60:61:62:63:64,id=idpBvu3g,netdev=idTjoQTn,bus=pci.0,addr=05 \
-netdev tap,id=idTjoQTn

2.Change mac address in guest
#ip link set eth0 addr 9a:60:61:62:63:68

3.Check the mac address in guest and qemu monitor
Before Mac change:
(qemu) info network 
Devices not on any VLAN:
  idTjoQTn: ifname=tap0,script=/etc/qemu-ifup,downscript=/etc/qemu-ifdown peer=idpBvu3g
  idpBvu3g: model=e1000,macaddr=9a:60:61:62:63:64 peer=idTjoQTn
After Mac change:
(qemu) info network 
Devices not on any VLAN:
  idTjoQTn: ifname=tap0,script=/etc/qemu-ifup,downscript=/etc/qemu-ifdown peer=idpBvu3g
  idpBvu3g: model=e1000,macaddr=9a:60:61:62:63:64 peer=idTjoQTn

The same result with rtl8139 NIC.

-----------------------------------------------------------------------------
Verify this bug in following versions:
Host:
    2.6.32-672.el6.x86_64
    qemu-kvm-0.12.1.2-2.498.el6.x86_64
Guest:
    2.6.32-671.el6.x86_64

The same test steps:
Step3 results:
Before Mac change:
(qemu) info network 
Devices not on any VLAN:
  idTjoQTn: ifname=tap0,script=/etc/qemu-ifup,downscript=/etc/qemu-ifdown peer=idpBvu3g
  idpBvu3g: model=e1000,macaddr=9a:60:61:62:63:64 peer=idTjoQTn
After Mac change:
(qemu) info network 
Devices not on any VLAN:
  idTjoQTn: ifname=tap0,script=/etc/qemu-ifup,downscript=/etc/qemu-ifdown peer=idpBvu3g
  idpBvu3g: model=e1000,macaddr=9a:60:61:62:63:68 peer=idTjoQTn

Mac address can be change in qemu monitor, so this bug fixed well.

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.

https://rhn.redhat.com/errata/RHSA-2017-0621.html