lib: Move vlog.h to <openvswitch/vlog.h>

Signed-off-by: Thomas Graf <tgraf@noironetworks.com>
lib: Move vconn.h to <openvswitch/vconn.h>

Also moves definitions for struct vconn and pvconn to the public
header. The provider interface is kept private.

Signed-off-by: Thomas Graf <tgraf@noironetworks.com>
lib: Add test for library usage

This test covers inclusion of public header files and basic usage
of the API such as vlog.

Signed-off-by: Thomas Graf <tgraf@noironetworks.com>