Support added to receive data socket over control socket

Support added to receive data socket over control socket

As objected by QEMU upstream developers to use two different sockets for
starting/using of swtpm, This commit adds support for passing unix domain
socket over control channel.

The summary of the changes include:
 - Defined new control command CMD_SET_DATAFD, using this clients can send data
   socket.
 - set mlp.fd and mlp.flags outside of the mainloop
 - updated the testcases

Signed-off-by: Amarnath Valluri <amarnath.valluri@intel.com>