vshReadlineInit: Initialize only once

vshReadlineInit: Initialize only once

'vshReadlineInit' is called when interactive virsh is started but also
on each call to 'cmdComplete'. Calling it repeatedly (using the
'complete' command interactively, or multiple times in batch mode) leaks
the buffers for history file configuration.

Avoid multiple setups of this function by returning success in case the
history file config is already present.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>