vmspawn: Add comment explaining substring match in firmware_data_matches_machine()

The machine types in QEMU firmware descriptions are glob patterns
like "pc-q35-*", so we use strstr() substring matching to check if
our machine type is covered by a given firmware entry.

src/vmspawn/vmspawn-util.c

@@ -151,6 +151,11 @@ static bool firmware_data_matches_machine(const FirmwareData *fwd, const char *a
                 if (!streq((*t)->architecture, arch))
                         continue;
 
+                /* The machine types in firmware descriptions are glob patterns such as "pc-q35-*", but
+                 * we pass the short alias (e.g. "q35") as the machine type to QEMU as it always points to
+                 * the latest version. We can't use fnmatch() here because "q35" doesn't match the
+                 * "pc-q35-*" glob, so instead we use substring matching to check if our machine type
+                 * appears in the pattern. */
                 STRV_FOREACH(m, (*t)->machines)
                         if (strstr(*m, machine))
                                 return true;