block: Use common driver selection code for bdrv_open_file()

block: Use common driver selection code for bdrv_open_file()

This moves the bdrv_open_file() call a bit down so that it can use the
bdrv_open() code that selects the right block driver.

The code between the old and the new call site is either common code
(the error message for an unknown driver has been unified now) or
doesn't run with cleared BDRV_O_PROTOCOL (added an if block in one
place, whereas the right path was already asserted in another place)

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Benoit Canet <benoit@irqsave.net>