Unregister every registered application by MiniVM.

The MinivmMWI application was not being unregistered on unload and we were not
able to load again the module or reload it.

(closes issue #15174)
Reported by: junky
Patches:
      unregister_minivm_mwi.diff uploaded by junky (license 177)



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@196377 65c4cc65-6c06-0410-ace0-fbb531ad65f3