apparmor: fix ptrace rules with kernel 4.18

Due to kernel upstream change 338d0be4 ("apparmor: fix ptrace read check")
libvirt now hits apparmor denies like:
apparmor="DENIED" operation="ptrace" profile="/usr/sbin/libvirtd"
pid=4409 comm="libvirtd" requested_mask="read" denied_mask="read"
peer="libvirt-14e92a75-7668-4b97-8f92-322fc1b9c78a"

Extend the ptrace rule to also allow 'ptrace (read)' for libvirtd to work
with these newer kernels.

Fixes: https://bugs.launchpad.net/bugs/1788603

Reported-by: Thadeu Lima de Souza Cascardo <thadeu.cascardo@canonical.com>
Reviewed-by: Erik Skultety <eskultet@redhat.com>
Acked-by: Jamie Strandboge <jamie@canonical.com>
Signed-off-by: Christian Ehrhardt <christian.ehrhardt@canonical.com>