xenserver: Fix "ovs-vsctl get" call in vswitch-cfg-update.

The ovs-vsctl arguments were being passed as a single string instead of
broken up as if the shell had performed word splitting.  This fixes the
problem.

Without this commit, "unknown command 'get Open_vSwitch . managers';
use --help for help" appears in the system log.  Adding the commit
suppresses that message.

NIC-72.
Reported-by: Andy Southgate <andy.southgate@citrix.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>