qemu-pr-helper: use new libmultipath API

libmultipath has recently changed its API.  The new API supports multi-threaded
clients better.  Unfortunately there is no backwards-compatibility, so we just
switch to the new one.  Running QEMU compiled with the new library on the old
library will likely crash, while doing the opposite will cause QEMU not to
start at all (because udev, get_multipath_config and put_multipath_config
are undefined).
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

configure

@@ -3312,9 +3312,17 @@ if test "$mpath" != "no" ; then
 #include <mpath_persist.h>
 unsigned mpath_mx_alloc_len = 1024;
 int logsink;
+static struct config *multipath_conf;
+extern struct udev *udev;
+extern struct config *get_multipath_config(void);
+extern void put_multipath_config(struct config *conf);
+struct udev *udev;
+struct config *get_multipath_config(void) { return multipath_conf; }
+void put_multipath_config(struct config *conf) { }
+
 int main(void) {
-    struct udev *udev = udev_new();
-    mpath_lib_init(udev);
+    udev = udev_new();
+    multipath_conf = mpath_lib_init();
     return 0;
 }
 EOF

scsi/qemu-pr-helper.c

@@ -276,15 +276,26 @@ static void dm_init(void)
 
 /* Variables required by libmultipath and libmpathpersist.  */
 QEMU_BUILD_BUG_ON(PR_HELPER_DATA_SIZE > MPATH_MAX_PARAM_LEN);
+static struct config *multipath_conf;
 unsigned mpath_mx_alloc_len = PR_HELPER_DATA_SIZE;
 int logsink;
+struct udev *udev;
 
-static void multipath_pr_init(void)
+extern struct config *get_multipath_config(void);
+struct config *get_multipath_config(void)
 {
-    static struct udev *udev;
+    return multipath_conf;
+}
 
+extern void put_multipath_config(struct config *conf);
+void put_multipath_config(struct config *conf)
+{
+}
+
+static void multipath_pr_init(void)
+{
     udev = udev_new();
-    mpath_lib_init(udev);
+    multipath_conf = mpath_lib_init();
 }
 
 static int is_mpath(int fd)