Rename tunable event constants

Rename tunable event constants

For the new VIR_DOMAIN_EVENT_ID_TUNABLE event we have a bunch of
constants added

   VIR_DOMAIN_EVENT_CPUTUNE_<blah>
   VIR_DOMAIN_EVENT_BLKDEVIOTUNE_<blah>

This naming convention is bad for two reasons

  - There is no common prefix unique for the events to both
    relate them, and distinguish them from other event
    constants

  - The values associated with the constants were chosen
    to match the names used with virConnectGetAllDomainStats
    so having EVENT in the constant name is not applicable in
    that respect

This patch proposes renaming the constants to

    VIR_DOMAIN_TUNABLE_CPU_<blah>
    VIR_DOMAIN_TUNABLE_BLKDEV_<blah>

ie, given them a common VIR_DOMAIN_TUNABLE prefix.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>