Attributes are simple flags, associated with individual timers for their
whole lifetime.  They intended to be used to mark individual timers for
special handling when they fire.

New/init functions family in timer interface updated and refactored (new
'attribute' argument added, timer_list replaced with timer_list_group+type
combinations, comments improved to avoid info duplication).  Also existing
aio interface extended with attribute-enabled variants of functions,
which create/initialize timers.

Signed-off-by: Artem Pisarenko <artem.k.pisarenko@gmail.com>
Message-Id: <f47b81dbce734e9806f9516eba8ca588e6321c2f.1539764043.git.artem.k.pisarenko@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>