vswitch.ovsschema: Add datapath_types and port_types.

vswitch.ovsschema: Add datapath_types and port_types.

At startup enumerate datapath and port types and add this information to
the datapath_types and port_types columns in the ovsdb.

This allows an ovsdb client to query the datapath in order to determine
if certain datapath and port types exist. For example, by querying the
port_types column, an ovsdb client will be able to determine if this
instance of ovs-vswitchd was compiled with DPDK support.

Signed-off-by: Mark D. Gray <mark.d.gray@intel.com>
Signed-off-by: Billy O'Mahony <billy.o.mahony@intel.com>
[blp@nicira.com made several changes]
Signed-off-by: Ben Pfaff <blp@nicira.com>