- No more limit on the number of preseed parameters, by placing a preseed.cfg
  file inside the initrd, instead of passing them all via the command line.
  The kernel is usually limited to 32 parameters and it panics when there are
  too many.

- Dist-specific options.

- Recognize preseed aliases.

- late-command is run with '/bin/sh -e' and it must exit with EX_OK (0),
  otherwise the installer stops.

9p has no option to map uid/gid, which means that the user on the guest should
have the same ids than on the host to avoid issues when accessing mount points.

This is easier to create such user in 'vm.run' rather than in 'vm.install-*'.
In other words, this commit reverts to the previous behaviour that
'vm.install-*' does not create a normal user account by default.

Because revision specifies worktree state exactly, and if we have it
already, we don't need to fetch anything -- it is possible to
checkout/reset to the revision right away.

The reason for this patch is that fetching is sometimes not very fast or
even slow. For example today, gitlab.com had some outage and I've
developed this patch while waiting for it to come back.

/reviewed-by @jerome, @kazuhiko
/reviewed-on !1