parallels: split check for parallels format in parallels_open

parallels: split check for parallels format in parallels_open

and rework error path a bit. There is no difference at the moment, but
the code will be definitely shorter when additional processing will
be required for WithouFreSpacExt

Signed-off-by: Denis V. Lunev <den@openvz.org>
CC: Jeff Cody <jcody@redhat.com>
CC: Kevin Wolf <kwolf@redhat.com>
CC: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Jeff Cody <jcody@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>