vdpa/mlx5: fix event setup

vdpa/mlx5: fix event setup

The completion event mechanism should work only if at least one of the
virtqs has valid callfd to be notified on.

When all the virtqs works with poll mode, the event mechanism should not
be configured.

The driver didn't take it into account and crashed in the above case.

Do not configure event interrupt when all the virtqs are in poll mode.

Fixes: 8395927cdfaf ("vdpa/mlx5: prepare HW queues")

Signed-off-by: Matan Azrad <matan@mellanox.com>
Acked-by: Viacheslav Ovsiienko <viacheslavo@mellanox.com>