This controls how big the root filesystem is for the squashfs used in
the boot.iso, the default is 2GiB.
Note that larger rootfs sizes will require more memory and may cause the
build to fail.
Fedora now has a edk2 package so use the OVMF code from there. This also
adds using a copy of OVMF_VARS for each boot instead of reusing the one
provided by the package.