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 <[email protected]>
Cc: "Michael S. Tsirkin" <[email protected]>
Cc: Eduardo Habkost <[email protected]>
Cc: Igor Mammedov <[email protected]>
Cc: John Snow <[email protected]>
Cc: Kevin Wolf <[email protected]>
Cc: Paolo Bonzini <[email protected]>
Cc: Richard Henderson <[email protected]>
Cc: [email protected]
Cc: [email protected]
Reviewed-by: Michael S. Tsirkin <[email protected]>
Signed-off-by: Michael S. Tsirkin <[email protected]>