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

libsel4vm: Removed arm devices/vgic.h

The function 'vm_install_vgic' is no longer used externally as
its abstracted by the guest irqchip. Moved the prototype into
the private vgic header.

libsel4vm: Removed vgic dist datastructure extern

Removed the 'extern' for the vgic dist datastructure. This is
not necessary.

libsel4vm: Removed device struct in arm vgic

Removed the usage of the device struct in the arm vgic source as
it no longer exists in the library. This is replaced with a
private management datastructure.

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.

libsel4vm: Moved vgic src in main arm directory

Moved the arm vgic source into the arm arch sub-directory. This
implementation doesn't need to be significantly refactored and is
fine to exist with the main arm source.