tests: remove <readonly/> from IDE disks

Read-only IDE disks are not supported, but the error
is raised only when QEMU_CAPS_DRIVE_READONLY is set.

tests/qemuxml2argvdata/qemuxml2argv-seclabel-dynamic-override.xml

@@ -28,7 +28,6 @@
         </seclabel>
       </source>
       <target dev='hdb' bus='ide'/>
-      <readonly/>
       <address type='drive' controller='0' bus='0' target='0' unit='0'/>
     </disk>
     <controller type='usb' index='0'/>

tests/qemuxml2xmloutdata/qemuxml2xmlout-seclabel-dynamic-override.xml

@@ -28,7 +28,6 @@
         </seclabel>
       </source>
       <target dev='hdb' bus='ide'/>
-      <readonly/>
       <address type='drive' controller='0' bus='0' target='0' unit='0'/>
     </disk>
     <controller type='usb' index='0'>