Fix return type of the unload_plugins() function
Fix return type of the unload_plugins() function

It's not supposed to return anything.

vpodzime/libblockdev,atodorov/libblockdev,rhinstaller/libblockdev,rhinstaller/libblockdev,vpodzime/libblockdev,atodorov/libblockdev,rhinstaller/libblockdev,vpodzime/libblockdev,atodorov/libblockdev