virsh: modify vshStringToArray to duplicate the elements too

virsh: modify vshStringToArray to duplicate the elements too

At a slightly larger memory expense allow stealing of items from the
string array returned from vshStringToArray and turn the result into a
string list compatible with virStringSplit. This will allow to use the
common dealloc function.

This patch also fixes a few forgotten checks of return from
vshStringToArray and one memory leak.