virt-aa-helper: Allow RO access to /usr/share/edk2-ovmf

virt-aa-helper: Allow RO access to /usr/share/edk2-ovmf

When binary version of edk2 is distributed, the files reside
under /usr/share/edk2-ovmf as can be seen from Gentoo's ebuild
[1]. Allow virt-aa-helper to generate paths under that dir.

1: https://gitweb.gentoo.org/repo/gentoo.git/tree/sys-firmware/edk2-ovmf-bin/edk2-ovmf-bin-202202.ebuild
Resolves: https://bugs.gentoo.org/911786
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Andrea Bolognani <abologna@redhat.com>