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

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

Complete migration started in commit 017f333, 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

Implementation note: Unlike Boost.variant, std::variant lacks built-in
operator<< support. This commit implements the necessary operator<< for
AttributeValue, our specific std::variant instantiation, to preserve the
existing behavior.

Also, despite that `apply_visit()` calls can be replaced with `visit()`
without being qualified with `std::` because of ADL, we are taking this
opportunity to adding the `std::` prefix for better readability.

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