i386/pc: expose identifying the floppy controller

i386/pc: expose identifying the floppy controller

Factor out and expose the function to locate the floppy controller in
the system.
It will allow to dynamically populate the relevant objects in the ACPI
tables.

Signed-off-by: Roman Kagan <rkagan@virtuozzo.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Eduardo Habkost <ehabkost@redhat.com>
Cc: Igor Mammedov <imammedo@redhat.com>
Cc: John Snow <jsnow@redhat.com>
Cc: Kevin Wolf <kwolf@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Richard Henderson <rth@twiddle.net>
Cc: qemu-block@nongnu.org
Cc: qemu-stable@nongnu.org
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>