tools/libxl: Fix libxl_list_vcpu() following c/s 93e52d52

tools/libxl: Fix libxl_list_vcpu() following c/s 93e52d52

My reasoning regarding nr_cpus_out was wrong, as I had confused nr_cpus_out
with nr_vcpus_out.

Dario pointed this out, but the patch (having gained appropriate acks) got
committed before I could post a correction.

Noticed-by: Dario Faggioli <dario.faggioli@citrix.com>
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
CC: Ian Campbell <Ian.Campbell@citrix.com>
CC: Ian Jackson <Ian.Jackson@eu.citrix.com>
CC: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Dario Faggioli <dario.faggioli@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>