gluster: Correctly propagate errors when volume isn't accessible

The docs for glfs_init suggest that the function sets errno on every
failure. In fact it doesn't. As other functions such as
qemu_gluster_open() in the gluster block code report their errors based
on this fact we need to make sure that errno is set on each failure.

This fixes a crash of qemu-img/qemu when a gluster brick isn't
accessible from given host while the server serving the volume
description is.

Thread 1 (Thread 0x7ffff7fba740 (LWP 203880)):
 #0  0x00007ffff77673f8 in glfs_lseek () from /usr/lib64/libgfapi.so.0
 Torlus#1  0x0000555555574a68 in qemu_gluster_getlength ()
 Torlus#2  0x0000555555565742 in refresh_total_sectors ()
 Torlus#3  0x000055555556914f in bdrv_open_common ()
 Torlus#4  0x000055555556e8e8 in bdrv_open ()
 Torlus#5  0x000055555556f02f in bdrv_open_image ()
 Torlus#6  0x000055555556e5f6 in bdrv_open ()
 Torlus#7  0x00005555555c5775 in bdrv_new_open ()
 Torlus#8  0x00005555555c5b91 in img_info ()
 qemu#9  0x00007ffff62c9c05 in __libc_start_main () from /lib64/libc.so.6
 qemu#10 0x00005555555648ad in _start ()

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

block/gluster.c

@@ -207,6 +207,11 @@ static struct glfs *qemu_gluster_init(GlusterConf *gconf, const char *filename,
                          "volume=%s image=%s transport=%s", gconf->server,
                          gconf->port, gconf->volname, gconf->image,
                          gconf->transport);
+
+        /* glfs_init sometimes doesn't set errno although docs suggest that */
+        if (errno == 0)
+            errno = EINVAL;
+
         goto out;
     }
     return glfs;
@@ -482,7 +487,7 @@ static int qemu_gluster_create(const char *filename,
 
     glfs = qemu_gluster_init(gconf, filename, errp);
     if (!glfs) {
-        ret = -EINVAL;
+        ret = -errno;
         goto out;
     }