xen/arm: no need to set HCR_VI when using the vgic to inject irqs

xen/arm: no need to set HCR_VI when using the vgic to inject irqs

HCR_VI forces the guest to resume execution in IRQ mode and can actually
cause spurious interrupt injections.
The GIC is capable of injecting interrupts into the guest and causing it
to switch to IRQ mode automatically, without any need for the hypervisor
to set HCR_VI manually.

See ARM ARM B1.8.11 and chapter 5.4 of the Generic Interrupt Controller
Architecture Specification.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Julien Grall <julien.grall@linaro.org>
Acked-by: Ian Campbell <ian.campbell@citrix.com>