Description of problem:
libvirt attach nvdimm device with invalid access type

Version-Release number of selected component (if applicable):
libvirt-3.2.0-5.el7.x86_64
qemu-kvm-rhev-2.9.0-4.el7.x86_64
kernel-3.10.0-660.el7.x86_64

How reproducible:
100%

Steps to Reproduce:
1. Create nvdimm file on the host:
     # truncate -s 512M /tmp/nvdimm
     # truncate -s 256M /tmp/nvdimm2

2. Start a guest with one nvdimm device successfully.
 <maxMemory slots='16' unit='M'>2048</maxMemory>
  <memory unit='M'>1024</memory>
  <currentMemory unit='M'>512</currentMemory>
  <vcpu placement='static'>4</vcpu>
  .......
  <cpu mode='host-model' check='partial'>
    <model fallback='allow'/>
    <topology sockets='2' cores='1' threads='1'/>
    <numa>
      <cell id='0' cpus='0-1' memory='512' unit='M'/>
      <cell id='1' cpus='2-3' memory='512' unit='M'/>
    </numa>
  </cpu>
......
         <memory model='nvdimm' access='shared'>
      <source>
        <path>/tmp/nvdimm</path>
      </source>
      <target>
        <size unit='M'>512</size>
        <node>1</node>
        <label>
          <size unit='KiB'>256</size>
        </label>
      </target>
      <address type='dimm' slot='0'/>
    </memory>
  </devices>

3. Try to attach a nvdimm device with access='shar', attached the device, the access='(null)'
# cat  nvdimm.xml
   <memory model='nvdimm' access='shar'>
      <source>
        <path>/tmp/nvdimm2</path>
      </source>
      <target>
        <size unit='M'>256</size>
        <node>1</node>
         <label>
          <size unit='KiB'>128</size>
        </label>
      </target>
      <address type='dimm' slot='1'/>
    </memory>

# virsh attach-device r7-4 nvdimm.xml
Device attached successfully

# virsh dumpxml r7-4 | grep nvdimm -A 15
    <memory model='nvdimm' access='shared'>
      <source>
        <path>/tmp/nvdimm</path>
      </source>
      <target>
        <size unit='KiB'>524288</size>
        <node>1</node>
        <label>
          <size unit='KiB'>256</size>
        </label>
      </target>
      <alias name='nvdimm0'/>
      <address type='dimm' slot='0' base='0x100000000'/>
    </memory>
    <memory model='nvdimm' access='(null)'>
      <source>
        <path>/tmp/nvdimm2</path>
      </source>
      <target>
        <size unit='KiB'>262144</size>
        <node>1</node>
        <label>
          <size unit='KiB'>128</size>
        </label>
      </target>
      <alias name='nvdimm1'/>
      <address type='dimm' slot='1' base='0x11ffc0000'/>
    </memory>

Actual results:
Attached nvdimm device with access='shar' in step3, and the access='null' in dumpxml.

Expected results:
Get error in step3, as the access type should be 'shared' or 'private'.