pseries: fix migration-test and pxe-test

Commit 29cb4187 ("spapr: Set VSMT to smp_threads by default")
has introduced a new default value for VSMT that is not supported
by old kernels (before 4.13 kernel) and this breaks "make check"
on these kernels.

To fix that, explicitly set in the involved tests the value that was
used as the default value before the change.

Cc: Greg Kurz <groug@kaod.org>
Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Message-Id: <20191120142539.236279-1-lvivier@redhat.com>
Acked-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Tested-by: Greg Kurz <groug@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>