Give plugins a way how to check if they could run properly
Give plugins a way how to check if they could run properly

Some plugins require e.g. various tools to be available, so when loading them we
should give them a chance to check the environment for such tools or whatever
they require.

snbueno/libblockdev,dashea/libblockdev,rhinstaller/libblockdev,snbueno/libblockdev,rhinstaller/libblockdev,vpodzime/libblockdev,atodorov/libblockdev,rhinstaller/libblockdev,vpodzime/libblockdev,atodorov/libblockdev,atodorov/libblockdev,vpodzime/libblockdev,dashea/libblockdev