xend: Improve "cpus" parameter to be able to define CPU affinities for each VCPU

xend: Improve "cpus" parameter to be able to define CPU affinities for each VCPU

If we define the form of list of string to "cpus" parameter,
different CPU affinities are set for each VCPU as follows.

 # grep cpus /etc/xen/vm1
 cpus = ["2", "3"]
 vcpus = 2
 # xm create vm1
 Using config file "/etc/xen/vm1".
 Started domain vm1
 # xm vcpu-list vm1
 Name                                ID  VCPU   CPU State   Time(s)
 CPU Affinity
 vm1                                  1     0     2   r--       3.5 2
 vm1                                  1     1     3   -b-       3.2 3

If we define the form of string to "cpus" parameter as before,
a same CPU affinity is set for each VCPU as follows.

 # grep cpus /etc/xen/vm2
 cpus = "2,3"
 vcpus = 2
 # xm create vm2
 Using config file "/etc/xen/vm2".
 Started domain vm2
 # xm vcpu-list vm2
 Name                                ID  VCPU   CPU State   Time(s)
 CPU Affinity
 vm2                                  2     0     3   -b-       3.0
 2-3
 vm2                                  2     1     2   r--       2.6
 2-3

Signed-off-by: Masaki Kanno <kanno.masaki@jp.fujitsu.com>