Libvirt AppArmor profile blocks ptrace call from snap

[Giuseppe Petralia]

the exporter uses the libvirt interface.
When it is connected even if it is able to collect correctly all the metrics the kernel keeps logging:

```
Dec 15 11:43:57 peppepetra-XPS-13 kernel: [179654.196285] audit: type=1400 audit(1639565037.354:20192): apparmor="DENIED" operation="ptrace" profile="libvirtd" pid=12766 comm="libvirtd" requested_mask="read" denied_mask="read" peer="snap.prometheus-libvirt-exporter.daemon"
Dec 15 11:44:10 peppepetra-XPS-13 kernel: [179667.726772] audit: type=1400 audit(1639565050.886:20210): apparmor="DENIED" operation="ptrace" profile="libvirtd" pid=12766 comm="libvirtd" requested_mask="read" denied_mask="read" peer="snap.prometheus-libvirt-exporter.daemon"
Dec 15 11:44:16 peppepetra-XPS-13 kernel: [179673.103116] audit: type=1400 audit(1639565056.262:20211): apparmor="DENIED" operation="ptrace" profile="libvirtd" pid=12766 comm="libvirtd" requested_mask="read" denied_mask="read" peer="snap.prometheus-libvirt-exporter.daemon"
```

[Giuseppe Petralia (peppepetra)]

Related bug https://bugs.launchpad.net/snapd/+bug/1954935

[Giuseppe Petralia (peppepetra)]

Given that output doesn't change if ptrace is denied or allowed, I propose to fix that with a charm hook that needs to:

1. Update local/usr.sbin.libvirtd apparmor profile adding:
   deny ptrace (read) peer=snap.prometheus-libvirt-exporter.daemon,

2. Reload libvirtd apparmor profile:
   apparmor_parser -r /etc/apparmor.d/usr.sbin.libvirtd

Ref. https://wiki.debian.org/AppArmor/HowToUse#Edit_AppArmor_profiles

[JamesLin (jneo8)]

This bug don't require change in snap