KVM: arm/arm64: vgic: Fix kvm_device leak in vgic_its_destroy

kvm_device->destroy() seems to be supposed to free its kvm_device
struct, but vgic_its_destroy() is not currently doing this,
resulting in a memory leak, resulting in kmemleak reports such as
the following:

unreferenced object 0xffff800aeddfe280 (size 128):
  comm "qemu-system-aar", pid 13799, jiffies 4299827317 (age 1569.844s)
  [...]
  backtrace:
    [<00000000a08b80e2>] kmem_cache_alloc+0x178/0x208
    [<00000000dcad2bd3>] kvm_vm_ioctl+0x350/0xbc0

Fix it.

Cc: Andre Przywara <andre.przywara@arm.com>
Fixes: 1085fdc6

 ("KVM: arm64: vgic-its: Introduce new KVM ITS device")
Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>