Merge overlayfs fix, LTS kernel bump and test

In Linux 4.19 there has been a major rework of the overlayfs
implementation and it now opens files in lowerdir with O_NOATIME, which
in turn caused issues in our VM tests because the process owner of QEMU
doesn't match the file owner of the lowerdir.

The crux here is that 9p propagates the O_NOATIME flag to the host and
the guest kernel has no way of verifying whether that flag will lead to
any problems beforehand.

There is ongoing work to possibly fix this in the kernel, but it will
take a while until there is a working patch and consensus.

So in order to bring our default kernel back to 4.19 and of course make
it possible to run newer kernels in VM tests, I'm merging a small QEMU
patch as an interim solution, which we can drop once we have a working
fix in the next round of stable kernels.

Now we already had Linux 4.19 set as the default kernel, but that was
subsequently reverted in 048c36ccaa0add5e5de387e9de0d3775d3fdd10d
because the patch we have used was the revert of the commit I bisected a
while ago.

This patch broke overlayfs in other ways, so I'm also merging in a VM
test by @bachp, which only tests whether overlayfs is working, just to
be on the safe side that something like this won't happen in the future.

Even though this change could be considered a moderate mass-rebuild at
least for GNU/Linux, I'm merging this to master, mainly to give us some
time to get it into the current 19.03 release branch (and subsequent
testing window) once we got no new breaking builds from Hydra.

Cc: @samueldr, @lheckemann

Fixes: https://github.com/NixOS/nixpkgs/issues/54509
Fixes: https://github.com/NixOS/nixpkgs/issues/48828
Merges: https://github.com/NixOS/nixpkgs/pull/57641
Merges: https://github.com/NixOS/nixpkgs/pull/54508