arm/hvf: Add a WFI handler

arm/hvf: Add a WFI handler

Sleep on WFI until the VTIMER is due but allow ourselves to be woken
up on IPI.

In this implementation IPI is blocked on the CPU thread at startup and
pselect() is used to atomically unblock the signal and begin sleeping.
The signal is sent unconditionally so there's no need to worry about
races between actually sleeping and the "we think we're sleeping"
state. It may lead to an extra wakeup but that's better than missing
it entirely.

Signed-off-by: Peter Collingbourne <pcc@google.com>
Signed-off-by: Alexander Graf <agraf@csgraf.de>
Acked-by: Roman Bolshakov <r.bolshakov@yadro.com>
Reviewed-by: Sergio Lopez <slp@redhat.com>
Message-id: 20210916155404.86958-6-agraf@csgraf.de
[agraf: Remove unused 'set' variable, always advance PC on WFX trap,
        support vm stop / continue operations and cntv offsets]
Signed-off-by: Alexander Graf <agraf@csgraf.de>
Acked-by: Roman Bolshakov <r.bolshakov@yadro.com>
Reviewed-by: Sergio Lopez <slp@redhat.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>