tests/avocado: Skip tests that require a missing accelerator

tests/avocado: Skip tests that require a missing accelerator

If a test was tagged with the "accel" tag and the specified
accelerator it not present in the qemu binary, cancel the test.

We can now write tests without explicit calls to require_accelerator,
just the tag is enough.

Signed-off-by: Fabiano Rosas <farosas@suse.de>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Tested-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>