KVM: arm64: vgic-its: Add pointer to corresponding kvm_device

Going from the ITS structure to the corresponding KVM structure
would be quite handy at times. The kvm_device pointer that is
passed at create time is quite convenient for this, so let's
keep a copy of it in the vgic_its structure.

This will be put to a good use in subsequent patches.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>