feat: Add Xen support

[andreistan26]

Prerequisite checklist

• [ ] Read the contribution guidelines regarding submitting new changes to the project;
• [ ] Tested your changes against relevant architectures and platforms;
• [ ] Ran make fmt on your commit series before opening this PR;
• [ ] Updated relevant documentation.

Description of changes

GitHub-Closes: #990

[craciunoiuc]

Hey @andreistan26

any new updates here?

[andreistan26]

Hey @andreistan26

any new updates here?

Will finish it up this week

[craciunoiuc]

sure thing, no hurry, just wanted to get a status update

[craciunoiuc]

Can you also add this? thanks:

diff --git a/Makefile b/Makefile
index 2b5d0ac0..18f88386 100644
--- a/Makefile
+++ b/Makefile
@@ -272,6 +272,7 @@ buildenv-myself-full: ## OCI image containing the build environment for KraftKit
 buildenv-myself: ## OCI image containing KraftKit binaries.
 buildenv-qemu: ## OCI image containing a Unikraft-centric build of QEMU.
 buildenv-github-action: ## OCI image used when building Unikraft unikernels in GitHub Actions.
+buildenv-xen: ## OCI image containing a Unikraft-centric build of Xen.
 tools: ## Build all tools.
 kraft: ## The kraft binary.
 runu: ## The runu binary.