virt-aa-helper: add NVRAM store file for read/write

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>