p2v: show error dialog if virt-v2v fails (RHBZ#1167601)

Ensure the control connection exits with the same status code as
virt-v2v, and return an error from start_conversion if virt-v2v
failed.
generator: Implement Pointer arguments.

This implements Pointer arguments properly, at least for certain
limited definitions of "implements" and "properly".

'Pointer' as an argument type is meant to indicate a pointer passed to
an API.  The canonical example is the following proposed API:

  int guestfs_add_libvirt_dom (guestfs_h *g, virDomainPtr dom, ...);

where 'dom' is described in the generator as:

  Pointer ("virDomainPtr", "dom")

Pointer existed already in the generator, but the implementation was
broken.  It is not used by any existing API.

There are two basic difficulties of implementing Pointer:

(1) In language bindings there is no portable way to turn (eg.) a Perl
Sys::Virt 'dom' object into a C virDomainPtr.

(2) We can't rely on <libvirt/libvirt.h> being included (since it's an
optional dependency).

In this commit, we solve (2) by using a 'void *'.

We don't solve (1), really.  Instead we have a macro
POINTER_NOT_IMPLEMENTED which is used by currently all the non-C
language bindings.  It complains loudly and passes a NULL to the
underlying function.  The underlying function detects the NULL and
safely returns an error.  It is to be hoped that people will
contribute patches to make each language binding work, although in
some bindings it will always remain impossible to implement.
New(ish) API: guestfs_add_libvirt_dom.

This API already existed (as guestfs___add_libvirt_dom), and was used
by a few tools.

This commit changes it to a public API.

Note that for reasons outlined in the previous commit message, it is
impossible to call this from guestfish or from non-C language
bindings.
python: Improve harness for running Python tests.

It now understands exit code 77 == skip, amongst other improvements.
python: Implement Pointer ("virDomainPtr", _) (RHBZ#1075164).

This allows the Python binding of guestfs_add_libvirt_dom to work.

There is a regression test to ensure this keeps working.

Note this requires a patched libvirt-python, supporting the
c_pointer() method.
debug: add-domain: Dump XML of original domain.

Useful for debugging labelling issues.
launch: libvirt: Fix labelling of overlay files.

We had code (added for RHBZ#912499) which labels overlay files
correctly so that libvirt can read them.  Unfortunately this code was
broken by subsequent commits: the new backend setting for the
imagelabel is only copied to the 'data' struct during launch, but the
create_cow_overlay callback is called before launch (when adding
drives).

The fix is easy: ensure create_cow_overlay_libvirt checks for the
backend setting and initializes the 'data' struct.

This change also means we need to free (data->selinux_imagelabel)
before setting it in launch (and we do the same for
data->selinux_label, although that's not strictly necessary).

For background on this, see:
https://bugzilla.redhat.com/show_bug.cgi?id=912499#c10
python: Add a regression test for disk labelling.