selftests: KVM: Free the GIC FD when cleaning up in arch_timer

In order to correctly destroy a VM, all references to the VM must be
freed. The arch_timer selftest creates a VGIC for the guest, which
itself holds a reference to the VM.

Close the GIC FD when cleaning up a VM.

Signed-off-by: Oliver Upton <oupton@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20220406235615.1447180-4-oupton@google.com