9pfs: add tests using local fs driver The currently existing 9pfs test cases are all solely using the 9pfs 'synth' fileystem driver, which is a very simple and purely simulated (in RAM only) filesystem. There are issues though where the 'synth' fs driver is not sufficient. For example the following two bugs need test cases running the 9pfs 'local' fs driver: https://bugs.launchpad.net/qemu/+bug/1336794 https://bugs.launchpad.net/qemu/+bug/1877384 This patch set for that reason introduces 9pfs test cases using the 9pfs 'local' filesystem driver along to the already existing tests on 'synth'.