infect: Introduce and use parasite_blob_desc

The _desc is to describe where libcompel should task
parasite code from. For now the parasite is taken as
piece of memory, but more ways to load the code will
come soon.

Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>