Description of problem:
'virsh detach-device-alias' should report failure when detaching memory failed

Version-Release number of selected component (if applicable):
kernel-3.10.0-918.el7.x86_64
qemu-kvm-rhev-2.12.0-6.el7.x86_64
libvirt-4.5.0-1.el7.x86_64

How reproducible:
100%

Steps to Reproduce:
1. Prepare a running VM with the following configuration and the XML file
# virsh domstate b1
running

# virsh dumpxml b1
  <maxMemory slots='16' unit='KiB'>2124800</maxMemory>
  <memory unit='KiB'>1155072</memory>
  <currentMemory unit='KiB'>917504</currentMemory>
  <cpu mode='custom' match='exact' check='full'>
    <numa>
      <cell id='0' cpus='0-7' memory='512000' unit='KiB'/>
      <cell id='1' cpus='8-15' memory='512000' unit='KiB'/>
    </numa>
  </cpu>
  <device>
    <memory model='dimm'>
      <source>
        <nodemask>0</nodemask>
        <pagesize unit='KiB'>2048</pagesize>
      </source>
      <target>
        <size unit='KiB'>131072</size>
        <node>1</node>
      </target>
      <alias name='ua-9874229d-d867-4932-ad28-279e9a777f42'/>
      <address type='dimm' slot='1' base='0x100000000'/>
    </memory>
  </device>

# cat memory.xml 
    <memory model='dimm'>
      <source>
        <nodemask>0</nodemask>
        <pagesize unit='KiB'>2048</pagesize>
      </source>
      <target>
        <size unit='KiB'>131072</size>
        <node>1</node>
      </target>
      <alias name='ua-9874229d-d867-4932-ad28-279e9a777f42'/>
      <address type='dimm' slot='1' base='0x100000000'/>
    </memory>

2. Detach memory device through 'virsh detach-device'
# virsh detach-device b1 memory.xml 
error: Failed to detach device from memory.xml
error: operation failed: unplug of device was rejected by the guest

# virsh dumpxml b1 |grep "<memory m" -A11
    <memory model='dimm'>
      <source>
        <nodemask>0</nodemask>
        <pagesize unit='KiB'>2048</pagesize>
      </source>
      <target>
        <size unit='KiB'>131072</size>
        <node>1</node>
      </target>
      <alias name='ua-9874229d-d867-4932-ad28-279e9a777f42'/>
      <address type='dimm' slot='1' base='0x100000000'/>
    </memory>

3. Detach memory device through 'virsh detach-device-alias'
# virsh detach-device-alias b1 ua-9874229d-d867-4932-ad28-279e9a777f42
Device detach request sent successfully

# virsh dumpxml b1 |grep "<memory m" -A11
    <memory model='dimm'>
      <source>
        <nodemask>0</nodemask>
        <pagesize unit='KiB'>2048</pagesize>
      </source>
      <target>
        <size unit='KiB'>131072</size>
        <node>1</node>
      </target>
      <alias name='ua-9874229d-d867-4932-ad28-279e9a777f42'/>
      <address type='dimm' slot='1' base='0x100000000'/>
    </memory>

Actual results:
As step-3 shows

Expected results:
As step-2 shows

Additional info:
Bug 1320447 - [RFE] Report memory hotunplug failure​