x86, hvm: Move return-to-guest timer and interrupt cranking logic

x86, hvm: Move return-to-guest timer and interrupt cranking logic
outside of IRQ-safe context. This allows us to safely take
non-IRQ-safe spinlocks.

The drawback is that {vmx,svm}_intr_assist() now races new event
notifications delivered by IRQ or IPI. We close down this race by
having vcpu_kick() send a dummy softirq -- this gets picked up in
IRQ-sage context and will cause retry of *_intr_assist(). We avoid
delivering the softirq where possible by avoiding it when we are
running in the non-IRQ context of the VCPU to be kicked.

Signed-off-by: Keir Fraser <keir.fraser@citrix.com>