qemu-timer: introduce timer attributes

qemu-timer: introduce timer attributes

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>