memory: Store physical root MR in FlatView

memory: Store physical root MR in FlatView

Address spaces get to keep a root MR (alias or not) but FlatView stores
the actual MR as this is going to be used later on to decide whether to
share a particular FlatView or not.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Message-Id: <20170921085110.25598-10-aik@ozlabs.ru>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
(cherry picked from commit 89c177bbdd6cf8e50b3fd4831697d50e195d6432)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>