vsh: Use g_auto() for string lists returned in readline command/options generators

vsh: Use g_auto() for string lists returned in readline command/options generators

There are two functions that are used to generate completion
lists: vshReadlineCommandGenerator() for command names and
vshReadlineOptionsGenerator() for --options for given command.
Both return a string list, but may also fail while constructing
it. For that case, they call g_strfreev() explicitly, which is
needless since we have g_auto(GStrv).

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Jonathon Jongsma <jjongsma@redhat.com>