ANDROID: KVM: arm64: Check PGD alignment when creating a pVM

Martijn reported a hypervisor crash when providing pKVM with an
undersized PGD allocation. Indeed, although the size of the PGD
allocation at EL2 is not under host control, a smaller host-side
allocation can lead to providing pKVM with a misaligned PGD, which will
cause the guest stage-2 init to fail in a bad way. Specifically,
guest_s2_zalloc_pages_exact() expects a successful allocation from
hyp_alloc_pages(), which can only happen if the pool has been pre-filled
with a physically aligned high-order page.

In order to guarantee allocation success in this path, check the
host-provided PGD alignment early on.

Bug: 443668075
Change-Id: I170963edc5721cf368a506bcdc4b2d0cdb462f78
Fixes: ab3b93a3 ("FROMLIST: KVM: arm64: Add infrastructure to create and track pKVM instances at EL2")
Reported-by: Martijn Bogaard <martijnbogaard@google.com>
Signed-off-by: Quentin Perret <qperret@google.com>