memtune: change the way how we store unlimited value

There was a mess in the way how we store unlimited value for memory
limits and how we handled values provided by user.  Internally there
were two possible ways how to store unlimited value: as 0 value or as
VIR_DOMAIN_MEMORY_PARAM_UNLIMITED.  Because we chose to store memory
limits as unsigned long long, we cannot use -1 to represent unlimited.
It's much easier for us to say that everything greater than
VIR_DOMAIN_MEMORY_PARAM_UNLIMITED means unlimited and leave 0 as valid
value despite that it makes no sense to set limit to 0.

Remove unnecessary function virCompareLimitUlong.  The update of test
is to prevent the 0 to be miss-used as unlimited in future.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1146539

Signed-off-by: Pavel Hrdina <phrdina@redhat.com>

docs/formatdomain.html.in

@@ -798,7 +798,9 @@
         for <code>&lt;memory&gt;</code>.  For backwards
         compatibility, output is always in
         KiB.  <span class='since'><code>unit</code>
-        since 0.9.11</span></dd>
+        since 0.9.11</span>
+        Possible values for all *_limit parameters are in range from 0 to
+        VIR_DOMAIN_MEMORY_PARAM_UNLIMITED.</dd>
       <dt><code>hard_limit</code></dt>
       <dd> The optional <code>hard_limit</code> element is the maximum memory
         the guest can use. The units for this value are kibibytes (i.e. blocks