xl: implement and enable dryrun mode for `xl vcpu-pin'

xl: implement and enable dryrun mode for `xl vcpu-pin'

As it can be useful to see if the outcome of some complex vCPU
pinning bitmap specification looks as expected.

This also allow for the introduction of some automatic testing
and verification for the bitmap parsing code, as it happens
already in check-xl-disk-parse and check-xl-vif-parse.

In particular, to make the above possible, this commit also
changes the implementation of the vcpu-pin command so that,
instead of always returning 0, it returns an error if the
parsing fails.

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