xl: add support for vif rate limiting

xl: add support for vif rate limiting

The `rate` keyword specifies the rate at which the outgoing traffic
will be limited to. The default if this keyword is not specified
is unlimited.

The `rate` keyword supports an optional replenishment interval
parameter for specifying the granularity of credit replenishment.
It determines the frequency at which the vif transmission credit
is replenished. The default interval is 50ms.

For example:

        'rate=10Mb/s'
        'rate=250KB/s'
        'rate=1MB/s@20ms'

Signed-off-by: Mathieu Gagne <mgagne@iweb.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>