qemu-tool: Fix mixup of int64 and int64_t

Commit cbcfa0418f0c196afa765f5c9837b9344d1adcf3 (link the main loop and
its dependencies into the tools) introduced stray usages of int64.

Use int64_t instead.

Signed-off-by: Andreas Färber <afaerber@suse.de>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
