qemu-iotests: Add "_supported_os Linux" to 058

Other cases have this, and this test is not portable as well, as we want
to add "make check-block" to "make check", it shouldn't fail on Mac OS
X.

Reported-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

tests/qemu-iotests/058

@@ -87,6 +87,7 @@ trap "_cleanup; exit \$status" 0 1 2 3 15
 
 _supported_fmt qcow2
 _supported_proto file
+_supported_os Linux
 _require_command QEMU_NBD
 
 # Use -f raw instead of -f $IMGFMT for the NBD connection