allow fall back to default completion

allow fall back to default completion

This allows a fall back to the default completion, which completes
possible paths/files, if nothing can be generated from the PVE
completion handler ($print_bash_completion) or if the user starts
to entry a path.
This is especially useful for restore or create commands which can
take an archive file as argument.

The bash-completions get generated at each package build through the
respective Makefile, so to let this change come in effect the package
needs to be rebuild and installed (and the respective file from
/usr/share/bash-completion/completions/<pve cli tool> needs to be
sourced again (or simply open a new terminal))

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>