virt-aa-helper: don't deny writes to readonly mounts

virt-aa-helper: don't deny writes to readonly mounts

There is no need to deny writes on a readonly mount: write still
won't be accepted, even if the user remounts the folder as RW in
the guest as qemu sets the 9p mount as ro.

This deny rule was leading to problems for example with readonly /:
The qemu process had to write to a bunch of files in / like logs,
sockets, etc. This deny rule was also preventing auditing of these
denials, making it harder to debug.