tests/qtest: rename qtest_send_prefix and roll-up into qtest_send

tests/qtest: rename qtest_send_prefix and roll-up into qtest_send

qtest_send_prefix never actually sent something over the chardev, all
it does is print the timestamp to the QTEST_LOG when enabled. So
rename the function, make it static, remove the unused CharDev and
simplify all the call sites by handling that directly with
qtest_send (and qtest_log_send).

Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20250207153112.3939799-6-alex.bennee@linaro.org>