docker: dump 'config.log' if ./configure fails

Suggested-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NPhilippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: NDaniel P. Berrangé <berrange@redhat.com>
Message-Id: <20180315142713.30960-1-f4bug@amsat.org>
Signed-off-by: NFam Zheng <famz@redhat.com>

tests/docker/common.rc

@@ -30,7 +30,9 @@ build_qemu()
                  $@"
     echo "Configure options:"
     echo $config_opts
-    $QEMU_SRC/configure $config_opts && make $MAKEFLAGS
+    $QEMU_SRC/configure $config_opts || \
+        { cat config.log && test_fail "Failed to run 'configure'"; }
+    make $MAKEFLAGS
 }
 
 test_fail()