hotplug/NetBSD: check type of file to attach from params

hotplug/NetBSD: check type of file to attach from params

xend used to set the xenbus backend entry "type" to either "phy" or
"file", but now libxl sets it to "phy" for both file and block device.
We have to manually check for the type of the "param" field in order
to detect if we are trying to attach a file or a block device.

Signed-off-by: Christoph Egger <Christoph.Egger@amd.com>
Signed-off-by: Roger Pau Monne <roger.pau@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Committed-by: Ian Campbell <ian.campbell@citrix.com>