trace: forbid floating point types

trace: forbid floating point types

Only one existing trace event uses a floating point type.  Unfortunately
float and double cannot be supported since SystemTap does not have
floating point types.

Remove float and double from the whitelist and document this limitation.
Update the migrate_transferred trace event to use uint64_t instead of
double.

Cc: Dr. David Alan Gilbert <dgilbert@redhat.com>
Cc: Daniel P. Berrangé <berrange@redhat.com>
Cc: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Message-id: 20180621150254.4922-1-stefanha@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>