contrib/apparmor: Split profile into abstractions, use them

One day, libvirt might actually support running passt to provide
guest connectivity. Should libvirtd (or virtqemud) start passt, it
will need to access socket and PID files in specific locations, and
passt needs to accept SIGTERM in case QEMU fails to start after passt
is already started.

To make this more convenient, split the current profile into two
abstractions, for passt and for pasta, so that external programmes
can include the bits they need (and especially not include the pasta
abstraction if they only need to start passt), plus whatever specific
adaptation is needed.

For stand-alone usage of passt and pasta, the 'passt' profile simply
includes both abstractions, plus rules to create and access PID and
capture files in default or reasonable ($HOME) locations.

Tested on Debian with libvirt 9.0.0 together with a local fix to start
passt as intended, namely libvirt commit c0efdbdb9f66 ("qemu_passt:
Avoid double daemonizing passt"). This is an example of how the
libvirtd profile (or virtqemud abstraction, or virtqemud profile) can
use this:

  # support for passt network back-end
  /usr/bin/passt Cx -> passt,
  profile passt {
    /usr/bin/passt r,

    owner @{run}/user/[0-9]*/libvirt/qemu/run/passt/* rw,
    signal (receive) set=("term") peer=/usr/sbin/libvirtd,
    signal (receive) set=("term") peer=libvirtd,

    include if exists <abstractions/passt>
  }

translated:

- when executing /usr/bin/passt, switch to the subprofile "passt"
  (not the "discrete", i.e. stand-alone profile), described below.
  Scrub the environment (e.g. LD_PRELOAD is dropped)

- in the "passt" subprofile:

  - allow reading the binary

  - allow read and write access to PID and socket files

  - make passt accept SIGTERM from /usr/sbin/libvirtd, and
    libvirtd peer names

  - include anything else that's needed by passt itself

Suggested-by: Andrea Bolognani <abologna@redhat.com>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

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