Description of problem:
When graphic autoport is yes, update graphic device with same setting as running domain failed.

Version-Release number of selected component (if applicable):
# git describe 
v1.3.4-261-ga1ffedc

How reproducible:
100%

Steps to Reproduce:
1.Prepare a domain with graphic autoport=yes:
# virsh dumpxml rhel7
    <graphics type='spice' autoport='yes' listen='0.0.0.0' keymap='en-us'>
      <listen type='address' address='0.0.0.0'/>
    </graphics>

2.# virsh start rhel7

3.# virsh dumpxml rhel7
    <graphics type='spice' port='5900' autoport='yes' listen='0.0.0.0' keymap='en-us'>
      <listen type='address' address='0.0.0.0'/>
    </graphics>

4. Prepare a file containing same graphic setting as domain rhel7:
# cat /tmp/updategraphic
    <graphics type='spice' port='5900' autoport='yes' listen='0.0.0.0' keymap='en-us'>
      <listen type='address' address='0.0.0.0'/>
    </graphics>

5. Try to update graphic:
virsh # update-device rhel7 /tmp/updategraphic
error: Failed to update device from /tmp/updategraphic
error: Operation not supported: cannot change port settings on spice graphics


Actual results:
update device failed

Expected results:
update device successfully

Additional info:
When autoport='no', update can succeed.
    <graphics type='spice' port='5902' autoport='no' listen='0.0.0.0' keymap='en-us'>
      <listen type='address' address='0.0.0.0'/>
    </graphics>