ANDROID: iommu/arm-smmu-v3-kvm: Fix free_domain for VMs

With device assignment, a VM can die without freeing it's domains, so
at free_domain operation valid references for SMMUs might still exist.
Instead of asserting to panic, just free these references, as it's
guaranteed that the device is blocked.

Bug: 440103945
Change-Id: I875952a16f67c75c41a3a3631a9a710642337e81
Signed-off-by: Mostafa Saleh <smostafa@google.com>