This reverts commit e60483f2.

Recent commit cda4aa9a moved block backend creation before machine
property evaluation.  This broke block backends registering migration
blockers.  Commit e60483f2 fixed it by moving migration object
creation before block backend creation.  This broke migration with
Xen.  Turns out we need to configure the accelerator before we create
the migration object so that Xen's accelerator compat properties get
applied.  Revert the flawed commit.  This fixes the Xen regression,
but brings back the block backend regression.  The next commits will
fix it again.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20190401090827.20793-2-armbru@redhat.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>