- make add_solvable return a XSolvable
- make add_solvable return a XSolvable

JacksonIsaac/libsolv,jsilhan/libsolv,JacksonIsaac/libsolv,JacksonIsaac/libsolv,jsilhan/libsolv,jsilhan/libsolv,jsilhan/libsolv,JacksonIsaac/libsolv,jsilhan/libsolv,jsilhan/libsolv,JacksonIsaac/libsolv