s390x/pci: only limit DMA aperture if vfio DMA limit reported

s390x/pci: only limit DMA aperture if vfio DMA limit reported

If the host kernel lacks vfio DMA limit reporting, do not attempt
to shrink the guest DMA aperture.

Fixes: df202e3ff3 ("s390x/pci: shrink DMA aperture to be bound by vfio DMA limit")
Signed-off-by: Matthew Rosato <mjrosato@linux.ibm.com>
Message-ID: <20231110175108.465851-3-mjrosato@linux.ibm.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>
(cherry picked from commit 8011b508cf0ddbdbda03820f4fa6cd484a6d9aed)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>