fdc: fix media detection

We have to set up 'media_changed' after guest start so floppy driver
could detect that there is no media in drive. For this purpose we call
'fdctrl_change_cb' instead of 'fd_revalidate' in 'fdctrl_connect_drives'.
'fd_revalidate' is called inside 'fdctrl_change_cb'.

We still have to set default drive geometry in 'fd_revalidate' even
if there is no media in drive. When you try to open (windows) or mount (linux)
floppy the driver tries to seek on track 1. Linux guest stuck in loop then
kernel crashes and windows guest prints error message.

Signed-off-by: Pavel Hrdina <phrdina@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
