vfio/mdev: Avoid masking error code to EBUSY

Instead of masking return error to -EBUSY, return actual error
returned by the driver.

Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Signed-off-by: Parav Pandit <parav@mellanox.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>