tests: Clean up wait for event

We still use hacks like qmp("") to wait for an event, even though we
have qmp_eventwait() since commit 8fe941f7, and qmp_eventwait_ref()
since commit 7ffe3124.  Both commits neglected to convert all the
existing hacks.  Make up what they missed.

Bonus: gets rid of empty format strings.  A step towards compile-time
format string checking without triggering -Wformat-zero-length.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>
[thuth: dropped the hunks from the usb tests - not needed anymore]
Signed-off-by: Thomas Huth <thuth@redhat.com>