xl: error out if vNUMA specifies more vcpus than pcpus

xl: error out if vNUMA specifies more vcpus than pcpus

... but allow user to override that check by specifying maxvcpus= in xl
configuration file.

Note that the code is constructed such that the fallout is dealt with
after parsing. We can live with that because though it wastes a bit of
cpu cycles but it is still functionally correct and I would like to have
a clear split between parsing and dealing with fallouts.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Dario Faggioli <dario.faggioli@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>