ANDROID: KVM: arm64: pviommu: Mark allocated domain as busy

pkvm_guest_iommu_alloc_id will find the first available domain
from the bitmap and return it. However, never actually mark it
as set. So we always get the same domain id.

Bug: 429049173
Change-Id: I1a48ba2f6b44cc0d81d36848a9963f2703b11653
Signed-off-by: Mostafa Saleh <smostafa@google.com>