bhyve: implement ignore unknown MSRs feature
    
    Implement the MSRs ignore unknown reads and writes feature
    that's specified using:
    
      <features>
        ...
        <msrs unknown='ignore'>
        ...
      </features>
    
    in the domain XML.
    
    In bhyve, it's just passing '-w' command line argument to the bhyve(8)
    executable.
    
    Signed-off-by: Roman Bogorodskiy <bogorodskiy@gmail.com>
    Reviewed-by: Cole Robinson <crobinso@redhat.com>