virsh: Properly terminate string list in virshDomainInterfaceSourceModeCompleter()

virsh: Properly terminate string list in virshDomainInterfaceSourceModeCompleter()

A completer must return a NULL terminated list of strings, which
means that when dealing with enums, it has to allocate one
pointer more than the value of VIR_XXX_LAST. But this is not
honoured in virshDomainInterfaceSourceModeCompleter() leading to
out of bounds read.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Pavel Hrdina <phrdina@redhat.com>