tests/vm: Add workaround to consume console

tests/vm: Add workaround to consume console

This adds support to basevm.py so that we always
drain the console chars.  This makes use of
support added in an earlier commit that allows
QEMUMachine to use the ConsoleSocket.

This is a workaround we found was needed since
there is a known issue where QEMU will hang waiting
for console characters to be consumed.

We also added the option of logging the console to a file.
LOG_CONSOLE=1 will now log the output to a file.

Signed-off-by: Robert Foley <robert.foley@linaro.org>
Reviewed-by: Peter Puhov <peter.puhov@linaro.org>
Acked-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20200601211421.1277-10-robert.foley@linaro.org>
Message-Id: <20200701135652.1366-14-alex.bennee@linaro.org>