Detect location of qemu-bridge-helper

RedHat and Debian based distros use different locations

Reference: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=790935

configure.ac

@@ -2523,6 +2523,9 @@ AC_ARG_WITH([qemu-group],
 AC_DEFINE_UNQUOTED([QEMU_USER], ["$QEMU_USER"], [QEMU user account])
 AC_DEFINE_UNQUOTED([QEMU_GROUP], ["$QEMU_GROUP"], [QEMU group account])
 
+AC_PATH_PROG([QEMU_BRIDGE_HELPER], [qemu-bridge-helper], [/usr/libexec/qemu-bridge-helper],
+                  [/usr/libexec:/usr/lib/qemu])
+AC_DEFINE_UNQUOTED([QEMU_BRIDGE_HELPER], ["$QEMU_BRIDGE_HELPER"], [QEMU bridge helper])
 
 AC_ARG_WITH([macvtap],
   [AS_HELP_STRING([--with-macvtap],

src/qemu/qemu_conf.c

@@ -295,7 +295,7 @@ virQEMUDriverConfigPtr virQEMUDriverConfigNew(bool privileged)
             goto error;
     }
 
-    if (VIR_STRDUP(cfg->bridgeHelperName, "/usr/libexec/qemu-bridge-helper") < 0)
+    if (VIR_STRDUP(cfg->bridgeHelperName, QEMU_BRIDGE_HELPER) < 0)
         goto error;
 
     cfg->clearEmulatorCapabilities = true;