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

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

migrate from boost::variant to std::variant

Complete migration started in commit 017f3339c, replacing boost::variant with
std::variant throughout the librbd codebase. This change is part of our ongoing
effort to reduce third-party dependencies by leveraging C++ standard library
alternatives where possible.

Benefits include:
- Improved code readability and maintainability
- Reduced external dependency surface
- More consistent API usage with other components

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