hw/sd: add card-reparenting function

hw/sd: add card-reparenting function

Provide a new function sdbus_reparent_card() in sd core for reparenting
a card from a SDBus to another one.

This function is required by the raspi platform, where the two SD
controllers can be dynamically switched.

Signed-off-by: Clement Deschamps <[email protected]>
Reviewed-by: Peter Maydell <[email protected]>
Signed-off-by: Peter Maydell <[email protected]>
Message-id: 1488293711[email protected]
Message-id: 20170224164021[email protected]
Reviewed-by: Peter Maydell <[email protected]>
[PMM: added a doc comment to the header file; changed to
 use new behaviour of qdev_set_parent_bus()]
Signed-off-by: Peter Maydell <[email protected]>