thunderbolt: Add support for full PCIe daisy chains

Currently the software connection manager (tb.c) has only supported
creating a single PCIe tunnel, no PCIe device daisy chaining has been
supported so far. This updates the software connection manager so that
it now can create PCIe tunnels for full chain of six devices.

Because PCIe allows DMA and opens possibility for DMA attacks we change
security level to "user" meaning that PCIe tunneling requires that the
userspace authorizes the devices first. This makes it possible to block
PCIe tunneling completely while still allowing other types of tunnels to
be automatically created.

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>