vcpu_notify_event() previously took a boolean parameter which determines
whether the implementation should try to use a posted interrupt. On
arm64 and riscv, the implementation of vcpu_notify_event() is otherwise
identical to that of amd64.

With the aim of deduplicating vcpu state management code, introduce a
separate amd64-only function which tries to use posted interrupts. This
requires some duplication with vcpu_notify_event_locked(), but only a
little bit.

Then, fix up callers. No functional change intended.