trace: avoid SystemTap dtrace(1) warnings on empty files

trace: avoid SystemTap dtrace(1) warnings on empty files

target/hppa/trace-events only contains disabled events, resulting in a
trace-dtrace.dtrace file that says "provider qemu {}".  SystemTap's
dtrace(1) tool prints a warning when processing this input file.

This patch avoids the error by emitting an empty file instead of
"provider qemu {}" when there are no enabled trace events.

Fixes: 23c3d569f44284066714ff7c46bc4f19e630583f ("target/hppa: add TLB trace events")
Reported-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Liam Merwick <liam.merwick@oracle.com>
Message-id: 20190321170831.6539-3-stefanha@redhat.com
Message-Id: <20190321170831.6539-3-stefanha@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>