Pvgrub with lz4 compressed kernels

["John S" <johnsutherland@xxxxxxxxxxxxx>]

I am running PV guests on xen 4.11 with pvgrub. The recent releases of ubuntu 
(>19.04) look to have switched to lz4 compressed kernels. These kernels work 
fine with direct boot, but under pvgrub and pvgrub2 they don't seem bootable.

Most recent release of ubuntu focal 20.04. Guest kernel 5.4.0-28.
```
error: not xen image.                [ vmlinuz-5.4.0-28-gen  10B  0%  0.17B/s ]
error: you need to load the kernel first.

Press any key to continue...
```

The same setup works with an older kernel (e.g. 4.19)

I notice there was an issue for this patched into xen itself, has anyone got 
these kernels working with pvgrub?

Cheers!
John S