nfs: Use QAPI options in nfs_client_open()

Using the QAPI visitor to turn all options into QAPI BlockdevOptionsNfs
simplifies the code a lot. It will also be useful for implementing the
QAPI based .bdrv_co_create callback.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>