fix AppArmor driver for pipe character devices

fix AppArmor driver for pipe character devices

The AppArmor security driver adds only the path specified in the domain
XML for character devices of type 'pipe'. It should be using <path>.in
and <path>.out. We do this by creating a new vah_add_file_chardev() and
use it for char devices instead of vah_add_file(). Also adjust
valid_path() to accept S_FIFO (since qemu chardevs of type 'pipe' use
fifos). This is https://launchpad.net/bugs/832507