xl: Allow use of /dev/null with xl create to enable command-line definition

xl: Allow use of /dev/null with xl create to enable command-line definition

xm allows specifying /dev/null as the domain configuration argument to its
create option; add same functionality to xl. xl treats the configuration
argument /dev/null as a special case.  This allows specifying an entire
domain configuration on the command line.

Signed-off-by: W. Michael Petullo <mike@flyn.org>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
[ ijc -- ported to xen-unstable from 4.1 ]
Committed-by: Ian Campbell <ian.campbell@citrix.com>