virpci: don't fail VFIO passthrough when modules.alias is missing

When modules.alias is not available (e.g. monolithic kernel),
virPCIDeviceFindBestVFIOVariant() would fail, causing the entire
PCI device detach to abort.

Instead, log a warning and return success with no variant found,
allowing the caller to fall back to the generic vfio-pci driver.

Signed-off-by: Baptiste Daroussin <baptiste.daroussin@ovhcloud.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>

src/util/virpci.c

@@ -1549,6 +1549,12 @@ virPCIDeviceFindBestVFIOVariant(virPCIDevice *dev,
 
     uname(&unameInfo);
     modulesAliasPath = g_strdup_printf("/lib/modules/%s/modules.alias", unameInfo.release);
+    if (!virFileExists(modulesAliasPath)) {
+        /* on monolithic kernel this file does not exist */
+        VIR_DEBUG("modules.alias not available (%s), skipping VFIO variant detection",
+                 modulesAliasPath);
+        return 0;
+    }
     if (virFileReadAll(modulesAliasPath, 8 * 1024 * 1024, &modulesAliasContent) < 0)
         return -1;