kvm: Free current_cpu identifier

Since CPU loops are done as last step in kvm_{insert,remove}_breakpoint()
and kvm_remove_all_breakpoints(), we do not need to distinguish between
invoking CPU and iterated CPUs and can thereby free the identifier for
use as a global variable.

Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>