Tunnelled migration doesn't require any extra network connections
beside the libvirt daemon.  It's capable of strong encryption and the
default option of openstack-nova.

This patch adds the tunnelled migration(Tunnel3params) support to
libxl.  On the source side, the data flow is:

 * libxlDoMigrateSend() -> pipe libxlTunnel3MigrationFunc() polls pipe
 * out and then write to dest stream.

While on the destination side:
 * Stream -> pipe -> 'recvfd of libxlDomainStartRestore'

The usage is the same as p2p migration, execpt adding one extra
'--tunnelled' to the libvirt p2p migration command.
Signed-off-by: NBob Liu <bob.liu@oracle.com>
Signed-off-by: NJoao Martins <joao.m.martins@oracle.com>