Support mapping of empty pages during TVM's execution

[wojciechozga]

Description of the changes

To promote a VM to TVM, code and data pages must be preloaded to allow the security monitor to copy pages from non-confidential to confidential memory. Previous KVM CoVE patches were preloading all pages, including empty pages, what was leading to slow TVM boot times. Since majority of initial TVM pages are empty and are lazily loaded when TVM accesses them, this PR adjusts KVM CoVE patches to preload only "measured pages" defined by a VMM. The security monitor supports then mapping of empty pages during TVM runtime. Systems with real-time requirements should implement a mechanism inside a TVM to load/store from every page they will use in runtime to ensure that no G-stage mappings with TLB shutdown is required when TVM enters into operational state.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Formal verification
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Refactorization (non-breaking change which improves code quality)