Description of problem:
when there is no numatune for memory, 'auto' placement vcpu will add 'auto' placement memory numatune automatically.
But user specified memnode for each guest NUMA node in numatune, when 'auto' placement vcpu without memory placement. The libvirtd will add memory placement to 'auto' and remove memnode.

Although this configuration doesn't align vcpus, but it should be valid.

Version-Release number of selected component (if applicable):
libvirt-1.2.8-15.el7.x86_64
qemu-kvm-rhev-2.1.2-21.el7.x86_64
3.10.0-223.el7.x86_64

How reproducible:
100%

Steps to Reproduce:
1.  prepare guest xml
# cat dummy.xml
<domain type='qemu'>
  <name>dummy</name>
  <memory unit='KiB'>2097152</memory>
  <vcpu placement='auto'>4</vcpu>
  <numatune>
    <memnode cellid="0" mode="strict" nodeset="1"/>
    <memnode cellid="1" mode="preferred" nodeset="0"/>
  </numatune>
  <os>
    <type arch='x86_64' machine='pc'>hvm</type>
  </os>
  <cpu>
    <numa>
      <cell id='0' cpus='0-1' memory='1048576'/>
      <cell id='1' cpus='2-3' memory='1048576'/>
    </numa>
  </cpu>
</domain>

# virsh define dummy.xml


2. then check xml, the <memnode> is lost
# virsh dumpxml dummy
<domain type='qemu'>
  <name>dummy</name>
  <uuid>1fd33000-27cb-425c-94f9-c7454914acdb</uuid>
  <memory unit='KiB'>2097152</memory>
  <currentMemory unit='KiB'>2097152</currentMemory>
  <vcpu placement='auto'>4</vcpu>
  <numatune>
    <memory mode='strict' placement='auto'/>
  </numatune>
  <os>
    <type arch='x86_64' machine='pc-i440fx-rhel7.1.0'>hvm</type>
    <boot dev='hd'/>
  </os>
  <cpu>
    <numa>
      <cell id='0' cpus='0-1' memory='1048576'/>
      <cell id='1' cpus='2-3' memory='1048576'/>
    </numa>
  </cpu>
  <clock offset='utc'/>
  <on_poweroff>destroy</on_poweroff>
  <on_reboot>restart</on_reboot>
  <on_crash>destroy</on_crash>
  <devices>
    <emulator>/usr/libexec/qemu-kvm</emulator>
    <controller type='usb' index='0'>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x01' function='0x2'/>
    </controller>
    <controller type='pci' index='0' model='pci-root'/>
    <memballoon model='virtio'>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x02' function='0x0'/>
    </memballoon>
  </devices>
</domain>

Expect result:
memnode is an other memory placement configuration, 'auto' vcpu placement should not remove it.