{emulators,sysutils}/xen-{kernel,tools}: update to 4.21

Update to latest stable version. Fetch QEMU from the release tarball,
building from git is getting more and more complex, and we don't usually
have a delta of patches on top of QEMU that would dissaper if using git.

Sponsored by: Citrix Systems R&D
Approved by: bapt (implicit)