For Device parameters we expect a block device name. However we were
only testing for "/dev/..." and so chardevs (from the appliance) could
be passed here, resulting in strange effects. This adds a function
is_device_parameter which tests for a valid block device name.
For Dev_or_Path parameters much the same, except we can also use the
is_device_parameter function elsewhere in the daemon to distinguish if
we were called with a device or path parameter. Previously we used a
simple test if the path begins with "/dev/...".
Reported by Mathieu Tarral.
