Improve preflight checks

[kaniini]

Quoting internal slack:

@azenla:

Oh wait, it does do a preflight check, but it's cursed what happens is it checks for xenstored, and xenstored doesn't launch unless it's on Xen so it checks if xen is installed by whether xenstored is running at the right socket / bus paths what we should do is actually preflight check xen by attempting to use the XEN_VERSION hypercalls, then check if xenstore is there, so we can tell the user the right error