bhyve: implement support for commandline args

bhyve: implement support for commandline args

Implement support for passing custom command line arguments
to bhyve using the 'bhyve:commandline' element:

  <bhyve:commandline>
    <bhyve:arg value='-newarg'/>
  </bhyve:commandline>

 * Define virDomainXMLNamespace for the bhyve driver, which
   at this point supports only the 'commandline' element
   described above,
 * Update command generation code to inject these command line
   arguments between driver-generated arguments and the vmname
   positional argument.

Signed-off-by: Roman Bogorodskiy <bogorodskiy@gmail.com>
Reviewed-by: John Ferlan <jferlan@redhat.com>