ncm-opennebula: bug fix enable/disable hyp

Several bug fixes #353 #349:
 * Included a new function to enable/disable hyp host
 * use constant replaced by readonly
 * Documentation changes
i would check here if there is more then one. if so, log error and continue with 
```perl
my @hostinstances = $one->get_hosts(qr{^$host$});
.. (check for more then one)...
my $hostinstance = $hostinstances->[0];
```