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

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: VM run interface

Refactored the run invocation into 'vm_run'. This updates the x86
and arm source to implement their runtime event loops when the
'vm_run' interface is used. The intented functionality of vm run
is such that it will start the vm catching vm exits. This will
either handle the exits internally or forward it to the user.