block/vdi: Error out immediately in vdi_create()

Currently, if an error occurs during the part of vdi_create() which
actually writes the image, the function stores -errno, but continues
anyway.

Instead of trying to write data which (if it can be written at all) does
not make any sense without the operations before succeeding (e.g.,
writing the image header), just error out immediately.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>