I'm sorry I cannot reproduce that, look:

# cat demo.xml 
<domain type='kvm'>
  <name>demo</name>
  <memory>32768</memory>
  <vcpu placement='static' cpuset='1-2' current='1'>4</vcpu>
  <os>
    <type arch='x86_64'>hvm</type>
  </os>
  <clock offset="utc"/>
  <on_poweroff>destroy</on_poweroff>
  <on_reboot>destroy</on_reboot>
  <on_crash>destroy</on_crash>
</domain>
# virsh create demo.xml 
Domain demo created from demo.xml

# pidof qemu-kvm
19490
# grep Cpus_allowed_list /proc/19490/task/*/status
/proc/19490/task/19490/status:Cpus_allowed_list:	1-2
/proc/19490/task/19492/status:Cpus_allowed_list:	1-2
# rpm -q libvirt
libvirt-0.10.2-6.el6.x86_64

Could you paste the whole XML, check if you have cgroups mounted and check what cpus have the tasks assigned in them?  Thanks

The point is the last step as follows.  What's the result in your test ?

# virsh emulatorpin rhel6.3
emulator: CPU Affinity
----------------------------------
       *: 0-3

emulatorpin should return the Cpus_allowed_list of qemu'main thread.  The expected result should be the same as Cpus_allowed_list. 
--------------------------------------------------------------------------------

# ll /cgroup/cpu/libvirt/qemu/rhel6.3/
total 0
--w--w--w- 1 root root 0 Oct 31 08:52 cgroup.event_control
-rw-r--r-- 1 root root 0 Oct 31 08:52 cgroup.procs
-rw-r--r-- 1 root root 0 Oct 31 08:52 cpu.cfs_period_us
-rw-r--r-- 1 root root 0 Oct 31 08:52 cpu.cfs_quota_us
-rw-r--r-- 1 root root 0 Oct 31 08:52 cpu.rt_period_us
-rw-r--r-- 1 root root 0 Oct 31 08:52 cpu.rt_runtime_us
-rw-r--r-- 1 root root 0 Oct 31 08:52 cpu.shares
-r--r--r-- 1 root root 0 Oct 31 08:52 cpu.stat
drwxr-xr-x 2 root root 0 Oct 31 08:52 emulator
-rw-r--r-- 1 root root 0 Oct 31 08:52 notify_on_release
-rw-r--r-- 1 root root 0 Oct 31 08:52 tasks
drwxr-xr-x 2 root root 0 Oct 31 08:52 vcpu0

# virsh dumpxml rhel6.3
<domain type='kvm' id='2'>
  <name>rhel6.3</name>
  <uuid>9669ae9b-4996-b7e7-5648-53a6df93d5aa</uuid>
  <memory unit='KiB'>1048576</memory>
  <currentMemory unit='KiB'>1048576</currentMemory>
  <vcpu placement='static' cpuset='0-2' current='1'>4</vcpu>
  <os>
    <type arch='x86_64' machine='rhel6.4.0'>hvm</type>
    <boot dev='hd'/>
  </os>
  <features>
    <acpi/>
    <apic eoi='on'/>
    <pae/>
  </features>
  <clock offset='utc'/>
  <on_poweroff>destroy</on_poweroff>
  <on_reboot>restart</on_reboot>
  <on_crash>restart</on_crash>
  <devices>
    <emulator>/usr/libexec/qemu-kvm</emulator>
    <disk type='file' device='disk'>
      <driver name='qemu' type='raw' cache='none'/>
      <source file='/var/lib/libvirt/images/kvm-rhel6.3-x86_64.img'/>
      <target dev='hda' bus='ide'/>
      <alias name='ide0-0-0'/>
      <address type='drive' controller='0' bus='0' target='0' unit='0'/>
    </disk>
    <controller type='usb' index='0'>
      <alias name='usb0'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x01' function='0x2'/>
    </controller>
    <controller type='ide' index='0'>
      <alias name='ide0'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x01' function='0x1'/>
    </controller>
    <interface type='network'>
      <mac address='52:54:00:c5:20:41'/>
      <source network='default'/>
      <target dev='vnet0'/>
      <alias name='net0'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x03' function='0x0'/>
    </interface>
    <serial type='pty'>
      <source path='/dev/pts/2'/>
      <target port='0'/>
      <alias name='serial0'/>
    </serial>
    <console type='pty' tty='/dev/pts/2'>
      <source path='/dev/pts/2'/>
      <target type='serial' port='0'/>
      <alias name='serial0'/>
    </console>
    <input type='mouse' bus='ps2'/>
    <graphics type='spice' port='5900' autoport='yes' listen='0.0.0.0' keymap='fdshjlf78970'>
      <listen type='address' address='0.0.0.0'/>
    </graphics>
    <sound model='ich6'>
      <alias name='sound0'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x04' function='0x0'/>
    </sound>
    <video>
      <model type='cirrus' vram='9216' heads='1'/>
      <alias name='video0'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x02' function='0x0'/>
    </video>
    <memballoon model='virtio'>
      <alias name='balloon0'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x05' function='0x0'/>
    </memballoon>
  </devices>
  <seclabel type='none'/>
</domain>

OK, that makes sense, I sent a fix upstream and I'm waiting for ACK.

Verify it as follows. The result is expected. Move its status to VERIFIED.

# rpm -q libvirt
libvirt-0.10.2-7.el6.x86_64

# virsh start rhel6.3
Domain rhel6.3 started

# virsh dumpxml rhel6.3 |grep vcpu
  <vcpu placement='static' cpuset='2-3' current='1'>7</vcpu>

# virsh emulatorpin rhel6.3
emulator: CPU Affinity
----------------------------------
       *: 2-3


# pidof qemu-kvm
30119

# grep Cpus_allowed_list /proc/30119/task/*/status
/proc/30119/task/30119/status:Cpus_allowed_list:	2-3
/proc/30119/task/30141/status:Cpus_allowed_list:	2-3
/proc/30119/task/30142/status:Cpus_allowed_list:	2-3

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-2013-0276.html