If PCI passthrough type is not supported, we should error out rather than
continue building the command line.

When starting a domain, the type has been already checked by
qemuPrepareHostdevPCICheckSupport() before building qemu command line,
so the problem doesn't emerge.

But when coverting a domain xml without specifying passthrough type explictly
to qemu arg, we will get a malformed command line.

the xml:

    <hostdev mode='subsystem' type='pci' managed='yes'>
      <source>
        <address domain='0x0001' bus='0x03' slot='0x00' function='0x0'/>
      </source>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x05' function='0x0'/>
    </hostdev>

the converted command line:

  -device ,host=0001:03:00.0,id=hostdev0,bus=pci.0,addr=0x5

After this patch, virsh gives an error message:

  virsh domxml-to-native qemu-argv /tmp/tmp.xml
  error: internal error: invalid PCI passthrough type 'default'
Signed-off-by: NHu Tao <hutao@cn.fujitsu.com>