pc: Fix floppy drives with if=none

Commit 63ffb564 broke floppy devices specified on the command line like
-drive file=...,if=none,id=floppy -global isa-fdc.driveA=floppy because it
relies on drive_get() which works only with -fda/-drive if=floppy.

This patch resembles what we're already doing for IDE, i.e. remember the floppy
device that was created and use that to extract the BlockDriverStates where
needed.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>