Report error when parsing character device target type

Report error when parsing character device target type

No useful error was being reported when an invalid character device
target type is specified in the domainXML. E.g.

    ...
    <console type="pty">
      <source path="/dev/pts/2"/>
      <target type="kvm" port="0"/>
    </console>
    ...

resulted in

error: Failed to define domain from x.xml
error: An error occurred, but the cause is unknown

With this small patch, the error is more helpful

error: Failed to define domain from x.xml
error: XML error: unknown target type 'kvm' specified for character device