Convert to SPDX license tags

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

trivial: Style changed files

libsel4vm: Moved VM tcb object into vcpu object

There is a strong association between a VCPU and the TCB it is
binded to. Thus the TCB object should exist in the context of the
VCPU it is associated with. In addition this makes the VM library
open to possible SMP extensions. This commit updates the
initialisation of the TCB object when creating a VCPU and
the means of accessing TCB objects.

libsel4vm: Enabled vm_run to exit to caller

Introduced vm exit constants that can be used to return to the
library caller (vmm) for handling specific vm exits. Updated the
exit handling functions and vm run loop to handle cases where it
may exit and return an exit reason to the vmm. This is an initial
change that will allow the vm to reduce policy and refactor out
various drivers from the library such that it can be handled and
managed by the vmm.

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.