block: Remove the obsolete -drive boot=on|off parameter

It's not working anymore since QEMU v1.3.0 - time to remove it now.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>