apparmor: allow libvirtd to call pygrub

apparmor: allow libvirtd to call pygrub

When using xen through libxl in Debian/Ubuntu it needs to be able to
call pygrub.

This is placed in a versioned path like /usr/lib/xen-4.11/bin.
In theory the rule could be more strict by rendering the libexec_dir
setting pkg-config can derive from libbxen-dev. But that would make
particular libvirt/xen packages version-depend on each other. It seems
more reasonable to avoid these versioned dependencies and use a wildcard
rule instead as it is already in place for libxl-save-helper.

Note: This change was in Debian [1] and Ubuntu [2] for quite some time
already.

[1]: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=931768
[2]: https://bugs.launchpad.net/ubuntu/+source/libvirt/+bug/1326003

Signed-off-by: Christian Ehrhardt <christian.ehrhardt@canonical.com>
Reviewed-by: Andrea Bolognani <abologna@redhat.com>
Acked-by: Jamie Strandboge <jamie@canonical.com>