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>
