nfs: Remove processed options from QDict

Commit c22a0345 QAPIfied option parsing in the NFS block driver, but
forgot to remove all the options we processed. Therefore, we get an
error in bdrv_open_inherit(), which thinks the remaining options are
invalid. Trying to open an NFS image will result in an error like this:

    Block protocol 'nfs' doesn't support the option 'server.host'

Remove all options from the QDict to make the NFS driver work again.

Cc: qemu-stable@nongnu.org
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-id: 20180516160816.26259-1-kwolf@redhat.com
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Jeff Cody <jcody@redhat.com>
Signed-off-by: Jeff Cody <jcody@redhat.com>