vsh: Fix 'stdin' closing in 'cmdComplete'

vsh: Fix 'stdin' closing in 'cmdComplete'

While the 'complete' command is meant to be hidden and used only for
the completion script, there's nothing preventing it being used in all
virsh modes.

This poses a problem as the command tries to close 'stdin' to avoid the
possibility that an auth callback would want to read the password.

In interactive mode this immediately terminates virsh and in
non-interactive mode it attempts to close it multiple times if you use
virsh in batch mode.

Fix the issues by using virOnce() to close it exactly once and do so
only in non-interactive mode.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>