ANDROID: KVM: arm64: Pre-populate host stage2

We will soon attempt to avoid any memory allocations from the host mem
abort path. In order to pave the way towards supporting this, let's
pre-populate the host stage-2 for the entire address space using as many
block mappings as possible. Some of these mappings may need to be
collapsed shortly after from fix_host_ownership() for example, so this
doesn't guarantee the absence of memory aborts altogether, but helps
getting the structure of the page-table in the right shape early on.

Bug: 264070847
Change-Id: Ib3ce25c893f779437ce473d64e08e8876870556c
Signed-off-by: Quentin Perret <qperret@google.com>