introspection: avoid white lines within <variablelist>

Looks like gdbus-codegen ends up making paragraphs and that breaks the
XML parsing:

      DOC   Building HTML
    ../../../../libmm-glib/generated/mm-gdbus-doc-org.freedesktop.ModemManager1.xml:60: parser error : Opening and ending tag mismatch: variablelist line 59 and para
    </para><para>          <varlistentry><term><literal>action</literal></term>
           ^
    ../../../../libmm-glib/generated/mm-gdbus-doc-org.freedesktop.ModemManager1.xml:110: parser error : Opening and ending tag mismatch: para line 110 and variablelist
    </para><para>        </variablelist>
                                        ^
    ../xml/ModemManager-dbus-reference.xml:93: element include: XInclude error : could not load ../../../../libmm-glib/generated/mm-gdbus-doc-org.freedesktop.ModemManager1.xml, and no fallback was found

introspection/org.freedesktop.ModemManager1.xml

@@ -50,7 +50,6 @@
         possible keys are:
 
         <variablelist>
-
           <varlistentry><term><literal>action</literal></term>
             <listitem>
               <para>
@@ -72,7 +71,6 @@
               </variablelist>
             </listitem>
           </varlistentry>
-
           <varlistentry><term><literal>name</literal></term>
             <listitem>
               <para>
@@ -82,7 +80,6 @@
               </para>
             </listitem>
           </varlistentry>
-
           <varlistentry><term><literal>subsystem</literal></term>
             <listitem>
               <para>
@@ -92,7 +89,6 @@
               </para>
             </listitem>
           </varlistentry>
-
           <varlistentry><term><literal>uid</literal></term>
             <listitem>
               <para>
@@ -104,7 +100,6 @@
               </para>
             </listitem>
           </varlistentry>
-
         </variablelist>
     -->
     <method name="ReportKernelEvent">