xenstore: modify init-xenstore-domain parameter syntax

xenstore: modify init-xenstore-domain parameter syntax

init-xenstore-domain takes only positional parameters today. Change
this to a more flexible parameter syntax allowing to specify additional
options or to omit some.

Today the supported usage is:

init-xenstore-domain <xenstore-kernel> <memory_mb> <flask-label>
                     [<ramdisk-file>]

Modify this to:

init-xenstore-domain --kernel <xenstore-kernel>
                     --memory <memory_mb>
                     [--flask <flask-label>]
                     [--ramdisk <ramdisk-file>]

The flask label is made optional in order to support xenstore domains
without the need of a flask enabled hypervisor.

Signed-off-by: Juergen Gross <jgross@suse.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>