Add new qtest command to set a device's inbound IRQs.
Enhance qtest error reporting.

When a FAIL response is sent from QEMU to a unit tester, print
the full contents of the message.  This change is kind of a hack.
Add new environment variables for debugging.

Add QTEST_QEMU_ARGS and QTEST_LOG_FILE enviornment variables.
Also add documentation.
Add some basic unit tests for STM32.

So far the tests only cover:
1) Basic GPIO functionality.
2) Flash alias.