CLIHandler: consider valid prefixes for completion

CLIHandler: consider valid prefixes for completion

With the change introduced in 57c0d0c69c687f2dff876aa81369622d0ae0a841
completion of partial commands stopped working (e.g. typing qm res<TAB><TAB>
yields nothing instead of 'reset resize resume rescan')

By returning undef as 'ref' 'print_bash_completion' has no reference of the
available (sub) commands anymore.

By checking if the current argument is a valid prefix of a possible command,
and conditionally not setting the 'ref' hash to undef, the functionality is
restored.

Additionally a small whitespace glitch was fixed.

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