tools: docker: Install a readable kernel for libguestfs-tools

The filesystem and EFI (capsule and secure boot) test setups try to use
guestmount and virt-make-fs respectively to prepare disk images to run
tests on. However, these libguestfs tools need a kernel image and fail
with the following message (revealed in debug/trace mode) if it can't
find one:

    supermin: failed to find a suitable kernel (host_cpu=x86_64).

    I looked for kernels in /boot and modules in /lib/modules.

    If this is a Xen guest, and you only have Xen domU kernels
    installed, try installing a fullvirt kernel (only for
    supermin use, you shouldn't boot the Xen guest with it).

This failure then causes these tests to be skipped in CIs. Install a
kernel package in the Docker containers so the CIs can run these
tests with libguestfs tools again (assuming the container is run with
necessary host devices and privileges). As this kernel would be only
used for virtualization, we can use the kernel package specialized for
that. On Ubuntu systems kernel images are not readable by non-root
users, so explicitly add read permissions with chmod as well.

Signed-off-by: Alper Nebi Yasak <alpernebiyasak@gmail.com>
Acked-by: Heinrich Schuchardt <xypron.glpk@gmx.de>