It makes sense for none of the callers to have negative value as an
output and, fortunately, if anyone tried defining domain with negative
memory or any other value parsed by virDomainParseScaledValue(), the
resulting value was 0.  That means we can error out during parsing as
it won't break anything.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1155843Signed-off-by: NMartin Kletzander <mkletzan@redhat.com>