Add a xenstore watch for the "speed" key...

Add a xenstore watch for the "speed" key...

...and allow the value to specify units.

The 'wire' speed advertised by XENNET to the Windows network stack can be
controlled by the value of a "speed" key in the xenstore frontend area.
Values of this key are currently interpreted as decimal gigabits-per-second.

This patch sets a watch on the key (meaning changes to it now take
immediate effect) and allows units to be specified: 'G/g' for gigabits-per-
second, 'M/m' for megabits-per-second, and 'K/k' for kilobit-per-second.
Thus, for example, '100M' means 100 megabits-per-second. If no unit is
specified then the value is assumed to be in gigabits-per-second, as before.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>