Older versions of qemu-img do not accept a '--output json' option

Signed-off-by: Nikos Skalkotos <skalkoto@grnet.gr>

The printed name of the sysprep tasks is constructed from the method
name by replacing the _ with " ". This can be used to overwrite it.

If you print the snapshotting message before disk.file is referenced,
you may end up outputting messages like this:

Snapshotting media source ... Examining source media `<image_file>' ...

Instead of:

Examining source media `<image_file>' ... looks like a block device
Snapshotting media source ... done

If you are a regular user, some root commands like modprobe may not be
in your path.

This exception does not exist in python 2.x

Start by waiting for 5sec before retrying and double the interval
after each retry.

The original value was 5min but was rather small. In Desktop versions
of windows the VirtIO installation may take longer.

When the button is pressed it displays further information about an
installed VirtIO driver.

Signed-off-by: Nikos Skalkotos <skalkoto@grnet.gr>

Feature image format support

Make sure the guestfs VM is killed before we launch the VM that
installs the VirtIO drivers.

snf-image-creator can work with different image formats nowadays.

Only use the device mapper when snapshotting block devices. If the
input media is a regular file, then create a qcow2 image and define
the original media as backing_file.

Use qemu-nbd to export a QEMU image as raw where needed

This used to be a class in the util module, but since this is only used
on the Image, it suits better there.

When snapshotting make sure that the size of the underlying block
device is equal to the input media's virtual size. If the input media
is not raw, the QEMU image driver may grow the underlying image. The
block device should be big enough to allow this kind of operations.

This can be used to act on the original media instead of a device-mapper
snapshot.

This resolves #10

There is no need to associate a loop device with the input media if we
are not snapshotting the input media. The loop device is only needed in
order to use the media with the device-mapper.

If a command needs root privileges use # as prompt. In any other case
use $.

Add a section on how to use snf-image-creator on non-raw format images

If qemu catches a signal, it terminates with 0 and prints a message in
stderr. Try to detect if the process has been terminated by a signal
by examining the stderr.

When the user picks a directory that contains VirtIO drivers, show him
the version of the discovered drivers in the confirmation box.

When working in wizard mode, give the user the ability to revert the
installation of the VirtIO drivers.

Don't treat shrink as a seperated task. Add it in the syspreps menu.

Change the sysprep enabling and disabling mechanism to always work
correct. The previous implementation was buggy and would erroneously
"remember" the executed syspreps on a reset.

Use _add_cleanup and _cleanup in do_sysprep

This is handy because this way we don't need to unlock the
Administrator account if it is locked. Any user that is in the
Administrators group can do the job.