virsh: Document bandwidth maximum more clearly

virsh: Document bandwidth maximum more clearly

Commit id '0e2d7305' modified the code to allow a negative value to be
supplied for the bandwidth argument of the various block virsh commands
and the migrate-setspeed; however, it failed to update the man page to
describe the "feature" whereby a very large value could be interpreted
by the hypervisor to mean maximum value allowed. Although initially
designed to handle a -1 value, the reality is just about any negative
value could be provided and essentially perform the same feature.