qmidev: New request API

Add qmidev_{make,send}_request, which make sending a request to the card
simpler, and fill in real, incrementing transaction IDs.

BUG=chromium-os:27301
TEST=no code calls this yet; next commit will use and test it

Change-Id: I0fd6bbe1ca93d19a63ba742379b1caec250ec98a