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Signed-off-by: Gerwin Klein <gerwin.klein@data61.csiro.au>

libsel4vm: Handle seL4_Fault_VPPIEvent on arm

The kernel now reservs PPI events and forwards them to user
level through a new vcpu fault type, seL4_Fault_VPPIEvent. We
extend the vcpu handlers for this new fault type that subsequently
injects the PPI into the VCPU.

libsel4vm: Added common HSR decode definitions

Created a new common arm include "processor.h" that has constant
defines and macros for decoding HSR values. This is common across
both aarch64 and arm_hyp. Updated arm sources to use these new
include (where revelant).

libsel4vm: Removed duplicated WFI HSR value

Removed a duplicated HSR WFI constant value. Updated the arm vm
vcpu fault handler to use the definition in the processor.h
include.

libsel4vm: Use enum for arm vm exits

Use an enum to manage the arm vm exit reasons rather than a series
of defines.

libsel4vm: Refactored arm vm event handler

Refactored the arm vm event/exit handler. This refactors the vm
exit handler code into a series of individual handlers for the
various vm exit cases. The overall intention with these changes
is to make the vm exit handler more modular and to reduce the
complexity of the vm_run function.