KVM: selftests: vgic_init kvm selftests fixup

Bring some improvements/rationalization over the first version
of the vgic_init selftests:

- ucall_init is moved in run_cpu()
- vcpu_args_set is not called as not needed
- whenever a helper is supposed to succeed, call the non "_" version
- helpers do not return -errno, instead errno is checked by the caller
- vm_gic struct is used whenever possible, as well as vm_gic_destroy
- _kvm_create_device takes an addition fd parameter

Signed-off-by: Eric Auger <eric.auger@redhat.com>
Suggested-by: Andrew Jones <drjones@redhat.com>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20210407135937.533141-1-eric.auger@redhat.com