xl: Support (by ignoring) xl migrate --live

xl: Support (by ignoring) xl migrate --live

xm migrate would do non-live migration (effectively, save, transfer
and restore) by default, unless you specified --live.

xl migrate always does live migration.  Honour (by ignoring) --live
for compatibility with old callers.  Document this.

(This patch should be backported as far as possible.)

Reported-by: Matthew Vernon <mcv21@cam.ac.uk>
Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
CC: Matthew Vernon <mcv21@cam.ac.uk>
Acked-by: Ian Campbell <ian.campbell@citrix.com>