[Libguestfs] v2v: docs: Removed vsyscall support in Debian kernels requiring workaround (RHBZ#1592061).

Debian kernels have disabled legacy vsyscall page support meaning they cannot runs binaries that predate c.2013. To enable it again you must add vsyscall=emulate to the kernel command line when booting. It's unclear why Debian disabled this as according to the documentation: Disabling this option saves about 7K of kernel size and possibly 4K of additional runtime pagetable memory. (presumably the "runtime pagetable memory" is per-process?) It seems to me like a rather poor trade-off for not being able to run binaries which are just 5 years old. While I was writing this patch I imagined a much more complex automatic workaround where we would try to detect if the host kernel had this problem and add the extra vsyscall=emulate parameter automatically. ‘./configure’ might carry a binary (for x86_64 only?) built to use the vsyscall page and try to run it under the host kernel. If it failed it would suggest using the workaround. This solution seems a bit complex, and so this patch simply updates the documentation which hopefully will make the problem discoverable. I don't know how widespread this problem is. We've not seen it in Fedora/RHEL kernels AFAIK. Rich.