libxl: support vif outgoing bandwidth QoS

libxl: support vif outgoing bandwidth QoS

The libxl_device_nic structure supports specifying an outgoing rate
limit based on a time interval and bytes allowed per interval. In xl
config a rate limit is specified as "<RATE>/s@<INTERVAL>". INTERVAL
is optional and defaults to 50ms.

libvirt expresses outgoing limits by average (required), peak, burst,
and floor attributes in units of KB/s. This patch supports the outgoing
bandwidth limit by converting the average KB/s to bytes per interval
based on the same default interval (50ms) used by xl.

Signed-off-by: Jim Fehlig <jfehlig@suse.com>