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: NStefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: NPhilippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: NJuan Quintela <quintela@redhat.com>
Message-id: 20180621150254.4922-1-stefanha@redhat.com
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>