iotests.py: Add qemu_io_silent

With qemu-io now returning a useful exit code, some tests may find it
sufficient to just query that instead of logging (and filtering) the
whole output.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 20180509194302.21585-5-mreitz@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>

tests/qemu-iotests/iotests.py

@@ -133,6 +133,15 @@ def qemu_io(*args):
         sys.stderr.write('qemu-io received signal %i: %s\n' % (-exitcode, ' '.join(args)))
     return subp.communicate()[0]
 
+def qemu_io_silent(*args):
+    '''Run qemu-io and return the exit code, suppressing stdout'''
+    args = qemu_io_args + list(args)
+    exitcode = subprocess.call(args, stdout=open('/dev/null', 'w'))
+    if exitcode < 0:
+        sys.stderr.write('qemu-io received signal %i: %s\n' %
+                         (-exitcode, ' '.join(args)))
+    return exitcode
+
 
 class QemuIoInteractive:
     def __init__(self, *args):