xl: introduce specific VCPU to PCPU mapping in config file

xl: introduce specific VCPU to PCPU mapping in config file

xm supports the following syntax (in the config file) for
specific VCPU to PCPU mapping:

cpus = "0-3,5,^1" # all vcpus run on cpus 0,2,3,5
cpus = ["2", "3"] # VCPU0 runs on CPU2, VCPU1 runs on CPU3

Allow for the same in xl.

This fixes what happened in changeset 54000bca7a6a, which
introduced suppot for the `cpus=` option within xl, but used
both the list (cpus=[2, 3]) and the string (cpus="2,3") syntax
for achieving the same behaviour (pin all guest's vcpus to the
pcpus in the list/string).

Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com>
Committed-by: Ian Campbell <ian.campbell@citrix.com>