qemu-iotests: Fix supported_oses check

There is a bug in the recently added sys.platform test, and we no longer
run python tests, because "linux2" is the value to compare here. So do a
prefix match. According to python doc [1], the way to use sys.platform
is "unless you want to test for a specific system version, it is
therefore recommended to use the following idiom":

if sys.platform.startswith('freebsd'):
    # FreeBSD-specific code here...
elif sys.platform.startswith('linux'):
    # Linux-specific code here...

[1]: https://docs.python.org/2.7/library/sys.html#sys.platformSigned-off-by: NFam Zheng <famz@redhat.com>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

tests/qemu-iotests/iotests.py

@@ -288,7 +288,7 @@ def main(supported_fmts=[], supported_oses=['linux']):
     if supported_fmts and (imgfmt not in supported_fmts):
         notrun('not suitable for this image format: %s' % imgfmt)
 
-    if sys.platform not in supported_oses:
+    if True not in [sys.platform.startswith(x) for x in supported_oses]:
         notrun('not suitable for this OS: %s' % sys.platform)
 
     # We need to filter out the time taken from the output so that qemu-iotest