x86-generic: add manifest aliases for x86-kvm and x86-xen_domu

Allow the autoupdater to work when updating from the removed targets.

targets/x86-generic

@@ -7,3 +7,5 @@ factory_image x86-generic combined-ext4 .img.gz
 factory_image x86-generic combined-ext4 .vdi
 factory_image x86-generic combined-ext4 .vmdk
 sysupgrade_image x86-generic combined-ext4 .img.gz
+manifest_alias x86-kvm
+manifest_alias x86-xen_domu