qemu-iotests: Test naming of throttling groups

Throttling groups are named using the 'group' parameter of the
block_set_io_throttle command and the throttling.group command-line
option. If that parameter is unspecified the groups get the name of
the block device.

This patch adds a new test to check the naming of throttling groups.

Signed-off-by: Alberto Garcia <berto@igalia.com>
Message-id: d87d02823a6b91609509d8bb18e2f5dbd9a6102c.1467986342.git.berto@igalia.com
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
