virt-aa-helper: add NVRAM store file for read/write
    
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    Some UEFI firmwares may want to use a non-volatile memory to store some
    variables.
    If AppArmor is enabled, and NVRAM store file is set currently
    virt-aa-helper does
    not add the NVRAM store file to the template. Add this file for
    read/write when
    this functionality is defined in domain XML.
    
    Signed-off-by: Peter Kieser <peter@kieser.ca>