pc: fix wrong CMOS values for floppy drives

Before commit 63ffb564dca94f8bda01ed6d209784104630a4d2, states for
floppy drives were calculated in fdc.c:fd_revalidate(). There it is
also considered whether a disk is inserted or not. The commit didn't copy
the logic completely to pc.c, which caused a regression.

Fix by adding the same check also to pc.c.

Reported-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Tested-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
