virsh: Add a helper to parse cpulist

virsh: Add a helper to parse cpulist

The 'virsh vcpupin' and 'virsh emulatorpin' commands use the same
code to parse the cpulist. This patch abstracts the same code as
a helper. Along with various code style fixes, and error improvement
(only error "Physical CPU %d doesn't exist" if the specified CPU
exceed the range, no "cpulist: Invalid format", see the following
for an example of the error prior to this patch).

% virsh vcpupin 4 0 0-8
error: Physical CPU 4 doesn't exist.
error: cpulist: Invalid format.