virsh: reject negative values for scaled integer
    
    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>