CI: add poky-altcfg in xen.yml for systemd image requirement

xen-image-minimal now requires systemd.  Add poky-altcfg (which has
systemd amongst other things) as an includes in the xen.yml file to work
around this.  Also, xen requires openssh instead of dropbear.  So,
override that entry.

Signed-off-by: Jon Mason <jon.mason@arm.com>

ci/xen.yml

@@ -4,10 +4,13 @@ header:
   version: 14
   includes:
     - ci/meta-virtualization.yml
+    - ci/poky-altcfg.yml
 
 local_conf_header:
   meta-virt: |
     DISTRO_FEATURES:append = " virtualization xen"
+  sshd: |
+    IMAGE_FEATURES:append = " ssh-server-openssh"
 
 target:
   - xen-image-minimal