conf: cpu: Fix parsing of vendor_id

conf: cpu: Fix parsing of vendor_id

This patch fixes a problem that vendor_id attribute can not be defined
when fallback attribute is not defined.

If I define domain xml like below:
<domain>
  <cpu>
    <model vendor_id='aaaabbbbcccc'>core2duo</model>
  </cpu>
</domain>

In dumpxml, vendor_id is not reflected:
<domain>
  <cpu mode='custom' match='exact'>
    <model fallback='allow'>core2duo</model>
  </cpu>
</domain>

The expected output is:
<domain>
  <cpu mode='custom' match='exact'>
    <model fallback='allow' vendor_id='aaaabbbbcccc'>core2duo</model>
  </cpu>
</domain>

If the fallback attribute and vendor_id attribute is defined at the same
time, it's reflected as expected.

Signed-off-by: Ken ICHIKAWA <ichikawa.ken@jp.fujitsu.com>