[host][xdc-server] Pass writes from client to xdc.

TEST= with usb handler changes
out/build-zircon/tools/xdc-server
out/build-zircon/tools/xdc-test-host -f out_file -u
fx shell xdc-test -f in_file -d

Change-Id: Id849771a198f4f8ccf9bc9934e379722937e7518

[host][xdc-server] Return transfers if client exits.

TEST= terminated xdc-test-host early and verified transfers
were returned.

Change-Id: I680e2b59775596c09fa871c2c9f87b067302ec0c

[host][xdc-server] Pass read transfers to clients.

Passes the transfer to the client of the matching stream id.

TEST= patch in usb handler changes,
out/build-zircon/tools/xdc-server
out/build-zircon/tools/xdc-test-host -f out_file
fx shell xdc-test -f in_file

Change-Id: I673b1fe7bd4887737e7177aec5f9d01b9aa89d5f

[dev][xdc-server] Send ctrl msgs to xdc device.

Notifies the xdc device side when a host stream goes on / offline.

TEST= patch local usb handler changes
out/build-zircon/tools/xdc-server
fx shell xdc-test -d -f file
out/build-zircon/tools/xdc-test-host -u -f file

Change-Id: Iacd9c1341490fc4a55e97646fde5fad5879a69e6

[host][xdc-server] Set host stream connect status.

The xdc device side sends a ctrl msg when a device stream goes on /
offline.

When a client registers that stream id, or currently has registered it,
we will set them as connected / not connected.

Later on we can use this to know when to queue more data from a client
to the xdc device.

TEST= patch in local usb handler changes
out/build-zircon/tools/xdcserver
fx shell xdc-test -u -f file
out/build-zircon/tools/xdc-test-host -d -f file

Stream id 1 should be connected.

Change-Id: I815141641cf76dd04a905333385a3d0f3ee06a4d

[dev][xdc-server] Process reads from UsbHandler.

Currently just handles ctrl messages - prints when a xdc device stream
goes on or offline.

Adds the UsbHandler::Transfer type which will wrap around a
libusb_transfer.

TEST= with local usb-handler changes patched in
out/build-zircon/tools/xdc-server
fx shell xdc-test -u -f file

Change-Id: I25ac882f79e6629b346a4467579886f54974a0bc

[host][xdc-server] Handle usb handler fd changes.

The usb handler will handle all libusb related activity.

The xdcserver will monitor for events on the usb handler fds.
For now just handle the event where the usb handler adds / removes fds.

The actual usb handler implementation is left to a future CL
once libusb is added to sysroot (BLD-214)

TEST= patched local usb handler changes
out/build-zircon/tools/xdc-server
Unplugged and plugged in debugging cable.

Change-Id: I11e16bde744dad8495f7b299d26ef4859d6ef611

[host][xdcserver] Beginnings of xdc host server.

The xdcserver will sit between host side clients and libusb.

Clients will open a socket to the xdc server and register for a stream
id.

In a future CL, client reads / writes after registering will
be transformed into libusb requests by the xdcserver, which are
then passed on to the xdc server on the device side and routed
to the appropriate stream id.

TEST= out/build-zircon/tools/xdc-server and
run test app added in
https://fuchsia-review.googlesource.com/c/zircon/+/174059

Change-Id: I2420317a209f42b56bf9968fce08eec3ae1ef775