util: string: Use VIR_AUTOSTRINGLIST instead of VIR_AUTOPTR(virString)

util: string: Use VIR_AUTOSTRINGLIST instead of VIR_AUTOPTR(virString)

Use of VIR_AUTOPTR and virString is confusing as it's a list and not a
single pointer. Replace it by VIR_AUTOSTRINGLIST as string lists are
basically the only sane NULL-terminated list we can have.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Erik Skultety <eskultet@redhat.com>