libxl, hotplug/Linux: default to phy backend for raw format file

libxl, hotplug/Linux: default to phy backend for raw format file

Modify libxl to allow raw format file to use phy backend.

For the hotplug script part, learn from NetBSD's block hotplug script --
test the path and determine the actual type of file (block device or
regular file) then use the actual type to determine which branch to run.

With these changes, plus the current ordering of backend preference
(phy > qdisk > tap), we will use phy backend for raw format file by
default.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>