vhost-vdpa: change name and polarity for vhost_vdpa_one_time_request()

vhost-vdpa: change name and polarity for vhost_vdpa_one_time_request()

The name vhost_vdpa_one_time_request() was confusing. No
matter whatever it returns, its typical occurrence had
always been at requests that only need to be applied once.
And the name didn't suggest what it actually checks for.
Change it to vhost_vdpa_first_dev() with polarity flipped
for better readibility of code. That way it is able to
reflect what the check is really about.

This call is applicable to request which performs operation
only once, before queues are set up, and usually at the beginning
of the caller function. Document the requirement for it in place.

Signed-off-by: Si-Wei Liu <si-wei.liu@oracle.com>
Message-Id: <1651890498-24478-7-git-send-email-si-wei.liu@oracle.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>