qa/run_xfstests_qemu.sh: disable 251, 260 and 288

qa/run_xfstests_qemu.sh: disable 251, 260 and 288

All three are skipped with virtio disks:

251	 [not run] FITRIM not supported on /dev/vdc
260	 [not run] FITRIM not supported on /dev/vdc
288	 [not run] FITRIM not supported on /dev/vdc

But 260 and 288 fail with ide disks, where discard defaults to on.  The
ancient kernel in our ubuntu-12.04.qcow2 doesn't support virtio discard
anyway so let's just disable them for consistency.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
(cherry picked from commit b274bca1b2dfd7a46f66d2075f20fba2d2b1f73b)