installer: Handle fs discovery more careful

Previously findfs was called multiple times to discover a device for a
label, but that result was not used further on, instead it was tried to
discovered the device for a label again, and that could fail.

Now the device is discovered once, and reused further on.

Change-Id: I7326807c44f6906b2f216eef8be5a2b25d34b232
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1156112
Signed-off-by: Fabian Deutsch <fabiand@fedoraproject.org>