Add support for instance disk IO control.

Part 3 for blueprint quota-instance-resource, by setting exposed params
"disk_read_bytes_sec, disk_read_iops_sec, disk_write_bytes_sec,
disk_write_iops_sec, disk_total_bytes_sec, disk_total_iops_sec"
through nova flavor-set, user can implement instance disk IO control
policy.
for example:
nova flavor-set m1.medium set disk_write_bytes_sec=10240000,
then the max disk write IO is 10MB/s for VM user.

partly implement blueprint quota-instance-resource
original reviews https://review.openstack.org/#/c/14151/
DocImpact

Change-Id: I8605acf61d7ddbfdaf6eee3ab135898df2449484

nova/tests/fake_imagebackend.py

@@ -41,7 +41,7 @@ class Backend(object):
                 pass
 
             def libvirt_info(self, disk_bus, disk_dev, device_type,
-                             cache_mode):
+                             cache_mode, extra_specs):
                 info = config.LibvirtConfigGuestDisk()
                 info.source_type = 'file'
                 info.source_device = device_type