bug#20965: qemu non-deterministic test failures

[Mark H Weaver <mhw@netris.org>]

I've seen at least two occurrences of a non-deterministic test failure
in qemu for x86_64 on hydra.  Both are essentially the same, except for
the random seed.  Here's the relevant log tail of the most recent one:

--8<---------------cut here---------------start------------->8---
GTESTER check-qtest-arm
socket_accept failed: Resource temporarily unavailable
**
ERROR:tests/libqtest.c:192:qtest_init: assertion failed: (s->fd >= 0 && s->qmp_fd >= 0)
GTester: last random seed: R02S68ad7b4ad35e64a4283ad867738ae469
/tmp/nix-build-qemu-2.3.0.drv-0/qemu-2.3.0/tests/Makefile:416: recipe for target 'check-qtest-aarch64' failed
make: *** [check-qtest-aarch64] Error 1
make: *** Waiting for unfinished jobs....
phase `check' failed after 117 seconds
builder for `/gnu/store/fv3kfpi3j7vg692ibkfaw6rvid1yabvj-qemu-2.3.0.drv' failed with exit code 1
@ build-failed /gnu/store/fv3kfpi3j7vg692ibkfaw6rvid1yabvj-qemu-2.3.0.drv - 1 builder for `/gnu/store/fv3kfpi3j7vg692ibkfaw6rvid1yabvj-qemu-2.3.0.drv' failed with exit code 1
guix build: error: build failed: build of `/gnu/store/fv3kfpi3j7vg692ibkfaw6rvid1yabvj-qemu-2.3.0.drv' failed
--8<---------------cut here---------------end--------------->8---

The excerpt above was from the first attempt of the following build:

  http://hydra.gnu.org/build/557468

After restarting the build, the second attempt succeeded.

I saved a full log from another similar failure on 10 June 2015, but I
neglected to save the other details.  Here's the relevant tail:

--8<---------------cut here---------------start------------->8---
GTESTER check-qtest-arm
socket_accept failed: Resource temporarily unavailable
**
ERROR:tests/libqtest.c:192:qtest_init: assertion failed: (s->fd >= 0 && s->qmp_fd >= 0)
GTester: last random seed: R02Sf42b4c6eb1ed085e2ce450c1f51f3505
/tmp/nix-build-qemu-2.3.0.drv-1/qemu-2.3.0/tests/Makefile:416: recipe for target 'check-qtest-aarch64' failed
make: *** [check-qtest-aarch64] Error 1
make: *** Waiting for unfinished jobs....
socket_accept failed: Resource temporarily unavailable
**
ERROR:tests/libqtest.c:192:qtest_init: assertion failed: (s->fd >= 0 && s->qmp_fd >= 0)
GTester: last random seed: R02S126fc36a13df8d61829c432073f209f0
/tmp/nix-build-qemu-2.3.0.drv-1/qemu-2.3.0/tests/Makefile:416: recipe for target 'check-qtest-arm' failed
make: *** [check-qtest-arm] Error 1
phase `check' failed after 155 seconds
builder for `/gnu/store/m0p5fjhy4jfqff7m720qw1ybv740a2jk-qemu-2.3.0.drv' failed with exit code 1
@ build-failed /gnu/store/m0p5fjhy4jfqff7m720qw1ybv740a2jk-qemu-2.3.0.drv - 1 builder for `/gnu/store/m0p5fjhy4jfqff7m720qw1ybv740a2jk-qemu-2.3.0.drv' failed with exit code 1
guix build: error: build failed: build of `/gnu/store/m0p5fjhy4jfqff7m720qw1ybv740a2jk-qemu-2.3.0.drv' failed
--8<---------------cut here---------------end--------------->8---

      Mark