ANDROID: KVM: arm64: Fix un-initialized iommu into mgt_allocator_ops

iommu_host_pool is only initialized after the iommu driver has (i.e.
kvm_iommu_ops is set). Meanwhile, mgt_allocator_ops can be called at any
time. There's therefore a need to check if the driver (and by extension
iommu_host_pool) has been initialized into each mgt_allocator_ops.

Bug: 277989609
Bug: 278749606
Bug: 330300498
Fixes: 60730d5e ("ANDROID: KVM: arm64: Add {refill,recliam}_hyp_pool")
Change-Id: I7afe1628ec4898e203c89b98f1708cea1593175d
Signed-off-by: Vincent Donnefort <vdonnefort@google.com>