refactor initial device creation for pacman

refactor initial device creation for pacman

`aab` installs `archlinux-keyring`, which in turn invokes `dirmngr` during
installation. `dirmngr` needs access (at least) to '/dev/null' (see [0]), which
`aab` only created afterwards (before populating the keyring). This lead to
`dirmngr` spinning (and filling the filesystem with a regular file
'${rootfs}/dev/null' containing error messages.

This patch changes the behavior of aab: it now creates the devices before
installing 'archlinux-keyring' and removes them after the keyring is populated.

In order to save one further `mkdir` call for the 'dev' directory, this was
appended to the `mkpath` invocation in ve_init.

[0] https://bbs.archlinux.org/viewtopic.php?id=222002

Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>