Some virsh commands have a size parameter, which is handled as scaled
integer. We don't have any *feature* that would allow to use '-1' as
maximum size, so it's safe to reject any negative values for those
commands.
Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1159171
Signed-off-by: Pavel Hrdina <phrdina@redhat.com>