missing dependency on linux-image-extra leads to unbootable domU using block devices

[Sébastien Chaumat]

Most of xen functionalities required modules that are in the linux-image-extra package but no dependency (even indirect) exists between xen packages and linux-image-extra-*-generic.

e.g :

As soon as block devices are uses in domU, xen requires the xen_blkback module which is in linux-image-extra package.

to reproduce the bug :

sdo apt-get install linux-image-extra-4.4.0-22-generic xen-system-amd64
create a domU configuration using a phy: disk (tested with lvm)

test.cfg :
-----
kernel = '/vmlinuz'
ramdisk = '/initrd.img'
vcpus = '1'
memory = '512'
root = '/dev/xvda2 ro'
disk = [
                  '/dev/vg/testlv,raw,xvda2,w',

              ]

name = 'test'

-----

boot the domU
sudo xl create test.cfg

stop the domU

sudo apt-get remove linux-image-extra-4.4.0-22-generic
reboot dom0 to ensure xen modules are unloaded
try to boot the domU

the domU fails to boot with
 libxl: error: libxl_device.c:952:device_backend_callback: unable to add device with path /local/domain/0/backend/vbd/1/51714

Please add this missing dependency.