KVM: arm64: Register 'selftest_vm' in the VM table

KVM: arm64: Register 'selftest_vm' in the VM table

In preparation for extending the pKVM page ownership selftests to cover
forceful reclaim of donated pages, rework the creation of the
'selftest_vm' so that it is registered in the VM table while the tests
are running.

Tested-by: Fuad Tabba <tabba@google.com>
Tested-by: Mostafa Saleh <smostafa@google.com>
Signed-off-by: Will Deacon <will@kernel.org>
Link: https://patch.msgid.link/20260330144841.26181-35-will@kernel.org
Signed-off-by: Marc Zyngier <maz@kernel.org>