block: Avoid qobject_get_try_str()

I'm about to remove qobject_get_try_str().  Use qstring_get_str()
instead.  Safe because the argument is known to be a QString here.

Cc: Kevin Wolf <kwolf@redhat.com>
Cc: Max Reitz <mreitz@redhat.com>
Cc: qemu-block@nongnu.org
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20201211171152.146877-11-armbru@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>