configure: Add explanation for --enable-xen-pci-passthrough

Signed-off-by: NAnthony PERARD <anthony.perard@citrix.com>
Reviewed-by: NMarkus Armbruster <armbru@redhat.com>
Signed-off-by: NStefano Stabellini <sstabellini@kernel.org>

configure

@@ -1588,7 +1588,7 @@ disabled with --disable-FEATURE, default is enabled if available:
   virtfs          VirtFS
   mpath           Multipath persistent reservation passthrough
   xen             xen backend driver support
-  xen-pci-passthrough
+  xen-pci-passthrough    PCI passthrough support for Xen
   brlapi          BrlAPI (Braile)
   curl            curl connectivity
   membarrier      membarrier system call (for Linux 4.14+ or Windows)