appliance: init: Initialize LVM in the same way as done on physical machines.

Running vgscan appears to be unnecessary.

The -aa option of vgchange activates LVs.

The --sysinit option disables locking.
builder: allow "no key" as key in Sigchecker

Additional way to distinguish no actual key available for signature
checking; make sure to not allow signing in such situation.
builder: use Sigchecker.gpgkey_type for the fingerprint

Use Sigchecker.gpgkey_type instead of just string as type in the
sources list; adapt the listing code (and its expected output) to that.

No behaviour change which eases a bit the addition of new sources with
other key types.
builder: add functions to read XDG_CONFIG_DIRS and XDG_CONFIG_PATH
builder: extract the default key to file

This is basically default_pubkey from sigchecker.ml, just extracted as
file. Not used right now, but will be in the future.
builder: switch sources to .conf files

Introduce and use simple .conf files to configure the sources of indexes
for virt-builder. The location of these files is in XDG_CONFIG_DIRS /
XDG_CONFIG_HOME, so it can be easily overridden.

There are three .conf(.in) files shipped with this commit:
- "test-index.conf.in" (in "test-config"), which points to the
  "test-index" index (used in tests only); the tests are adapted to
  point to the hierarchy containing this .conf
- "libguestfs.conf.in" (in "test-website"), which points to the local
  "index.asc" (i.e. the offline copy of the libguestfs.org index);
  run(.in) will point to the hierarchy providing this .conf
- "libguestfs.conf.in" (directly among the other sources), which points
  to the online "index.asc" and it is installed in sysconfdir, along
  with the key of this repository

The tests are adapted, other than to the different way to pick sources,
to the different output of --list, as "test-index" is not signed.
builder: remove VIRT_BUILDER_SOURCE and VIRT_BUILDER_FINGERPRINT

Drop these two environment variables, and the implicit hardcoded source
hosted at libguestfs.org.
This means all the sources must be provided as .conf files, or at each
invocation with --source.
builder: remove the default fingerprint/pubkey

Sigchecker can still import keys from the user's keyring, so there is no
need to hardcode fingerprint and key of the index hosted on
libguestfs.org (which is now pointed to using a .conf file).
builder: update documentation

Update the documentation according to the new .conf files structure, the
drop of VIRT_BUILDER_SOURCE and VIRT_BUILDER_FINGERPRINT, the drop of
hardcoded source and its key, etc.