xl: convert vcpuid to signed in main_vcpupin()

xl: convert vcpuid to signed in main_vcpupin()

As it needs to be, considering that we assign -1 to it (and
that, later, we check for it to be -1), to signify 'all vcpus'.

While at it, fix a coding style nit and improve error reporting.

Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com>
Reviewed-by: Don Slutz <dslutz@verizon.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>