libxl: add force option for xl vcpu-pin

libxl: add force option for xl vcpu-pin

In order to be able to undo a vcpu pin override in case of a kernel
driver error add a flag "-f" to the "xl vcpu-pin" command forcing the
hypervisor to undo the override.

Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Cc: Wei Liu <wei.liu2@citrix.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Dario Faggioli <dario.faggioli@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>