fdc: Make media change detection more robust

fdc: Make media change detection more robust

fdctrl_change_cb() gets called on a virtual media change via monitor.
It would be nice if host device block drivers called it on physical
media change, but they don't.

bdrv_media_changed() lets you poll for media change, but it returns
"don't know" except with block driver "host_floppy".

FDrive member media_changed gets set on device initialization and by
fdctrl_change_cb(), and cleared by fdctrl_media_changed().  Thus, it's
set on first entry to fdctrl_media_changed() since device
initialization or virtual media change.

fdctrl_media_changed() ignores media_changed unless
bdrv_media_changed() returns "don't know".  If we change media via
monitor (setting media_changed), and the new media's block driver
returns 0, we lose.  Fortunately, "host_floppy" always returns 1 on
first call.  Brittle.  Clean it up not to rely on it.

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