KVM: arm/arm64: Fix GICv4 ITS initialization issues

We should only try to initialize GICv4 data structures on a GICv4
capable system.  Move the vgic_supports_direct_msis() check inito
vgic_v4_init() so that any KVM VGIC initialization path does not fail
on non-GICv4 systems.

Also be slightly more strict in the checking of the return value in
vgic_its_create, and only error out on negative return values from the
vgic_v4_init() function.  This is important because the kvm device code
only treats negative values as errors and only cleans up in this case.
Errornously treating a positive return value as an error from the
vgic_v4_init() function can lead to NULL pointer dereferences, as has
recently been observed.

Acked-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>