libxl: extend pCPUs specification for vcpu-pin.

libxl: extend pCPUs specification for vcpu-pin.

Allow for "^<cpuid>" syntax while specifying the pCPUs list
during a vcpu-pin. This enables doing the following:

 xl vcpu-pin 1 1 0-4,^2

and achieving:

 xl vcpu-list
 Name                                ID  VCPU   CPU State   Time(s) CPU Affinity
 ...
 Squeeze_pv                           1     1    3   -b-       2.4  0-1,3-4
 ...

Negative ranges are also supported, such as "0-4,^1-2" to
mean "0,3-4"

Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>