iotests: Resolve test failures caused by hostname

`hostname -s` may output an errror:
hostname: Name or service not known
This causes all tests to fail for `make check-block`.

Suppress such error messages, letting the tests succeed.

Signed-off-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

tests/qemu-iotests/common.config

@@ -36,7 +36,7 @@ export LANG=C
 
 PATH=".:$PATH"
 
-HOST=`hostname -s`
+HOST=`hostname -s 2> /dev/null`
 HOSTOS=`uname -s`
 
 EMAIL=root@localhost    # where auto-qa will send its status messages