virt-aa-helper: Avoid duplicate when append rule

virt-aa-helper: Avoid duplicate when append rule

when a device is dynamically attached to a VM, and it needs a special
system access for apparmor, libvirt calls virt-aa-helper (with argument -F)
to append a new rule to the apparmor profile of the VM. virt-aa-helper does
not check for duplicate and blindly appends the rule to the profile. since
there is no rule removal when a device is detached, this can make the profile
grow in size if a big number of attach/detach operations are done and the
profile might hit the size limit and futur attach operations might dysfunction
because no rule can be added into the apparmor profile.

this patch tries to mitigate this issue by doing a duplicate check
when rules are appended into the profile. this fix does not guarantee
the absence of duplicates but should be enough to prevent the profile
to grow significantly in size and reach its size limit.

Signed-off-by: Hector CAO <hector.cao@canonical.com>
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>