virt-aa-helper: Ask for no deny rule for readonly disk elements

virt-aa-helper: Ask for no deny rule for readonly disk elements

Just because a disk element only requests read access doesn't mean
there may not be another readwrite request.

Using 'R' when creating the apparmor rule will prevent an implicit
write-deny rule to be created alongside. This does not mean write
is allowed but it would cause a denial message and probably more
relevant, allows to add write access later.

Resolves: https://gitlab.com/libvirt/libvirt/-/issues/622
Resolves: https://gitlab.com/libvirt/libvirt/-/issues/806
Bug-Ubuntu: https://bugs.launchpad.net/bugs/1554031
Bug-Ubuntu: https://bugs.launchpad.net/bugs/1692441
Signed-off-by: Christian Ehrhardt <christian.ehrhardt@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Wesley Hershberger <wesley.hershberger@canonical.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>