fish: Fix tests that specified qcow2 format, but passed a raw format disk.

In some tests we were specifying qcow2 as the image format when adding
a disk, but actually passing a raw format image.

Libguestfs previously did not detect this until guestfs_launch, but it
was still a bug to pass an incorrect format to guestfs_add_drive_opts.

It only worked because these tests never call guestfs_launch.

A later commit in this series will cause this to be detected
(sometimes) during guestfs_add_drive_opts.
drives: Centrally create overlays for read-only drives.

qemu has broken snapshot=on ... again.

Change the way that drives are created so that the backend no longer
has to use snapshot=on, <transient/> (which never worked), or UML's
corresponding COW-creation feature (also broken).

Instead of that, the src/drives.c code will create overlays when
required by calling into a new backend operation 'create_cow_overlay'.
This operation runs 'qemu-img create -b' or 'uml_mkcow' as determined
by the backend, and returns the name of the overlay.

The format of the overlay is still backend-specific because qemu needs
to use qcow2 and UML needs to use COW.

This patch also includes some factorization of the libvirt XML code.

This also drops the drv->priv (private per-drive data) field, since it
is no longer used by any backend.

This also moves the guestfs___drive_source_qemu_param utility
function, used & shared by the direct & libvirt backends only, into
src/launch-direct.c (from src/drives.c).