librbd: migrate from boost::variant to std::variant

librbd: migrate from boost::variant to std::variant

This change is part of a broader effort to reduce dependencies on
third-party libraries by leveraging C++ standard library alternatives.

Migrating from boost::variant to std::variant improves code readability
and maintainability while reducing external dependencies.

Signed-off-by: Kefu Chai <tchaikov@gmail.com>