qemu: domain: Move initialization of disk cachemode for <shareable> disks

The qemu command line generator code set disk caching of shareable disks
to 'none' when formatting the command line silently. Move this code to a
common place when preparing the domain definition for startup so that it
does not have to be duplicated.

The new test case shows that the actual cache mode will now be recorded
in the live XML definition.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>

src/qemu/qemu_command.c

@@ -1804,8 +1804,6 @@ qemuBuildDriveStr(virDomainDiskDefPtr disk,
     if (disk->cachemode) {
         virBufferAsprintf(&opt, ",cache=%s",
                           qemuDiskCacheV2TypeToString(disk->cachemode));
-    } else if (disk->src->shared && !disk->src->readonly) {
-        virBufferAddLit(&opt, ",cache=none");
     }
 
     if (disk->copy_on_read) {

src/qemu/qemu_domain.c

@@ -11891,11 +11891,22 @@ qemuDomainPrepareDiskSourceChain(virDomainDiskDefPtr disk,
 }
 
 
+static void
+qemuDomainPrepareDiskCachemode(virDomainDiskDefPtr disk)
+{
+    if (disk->cachemode == VIR_DOMAIN_DISK_CACHE_DEFAULT &&
+        disk->src->shared && !disk->src->readonly)
+        disk->cachemode = VIR_DOMAIN_DISK_CACHE_DISABLE;
+}
+
+
 int
 qemuDomainPrepareDiskSource(virDomainDiskDefPtr disk,
                             qemuDomainObjPrivatePtr priv,
                             virQEMUDriverConfigPtr cfg)
 {
+    qemuDomainPrepareDiskCachemode(disk);
+
     if (qemuDomainPrepareDiskSourceTLS(disk->src, cfg) < 0)
         return -1;
 

tests/qemuxml2startupxmloutdata/disk-drive-shared.xml

@@ -15,7 +15,7 @@
   <devices>
     <emulator>/usr/bin/qemu-system-i686</emulator>
     <disk type='block' device='disk'>
-      <driver name='qemu' type='raw'/>
+      <driver name='qemu' type='raw' cache='none'/>
       <source dev='/dev/HostVG/QEMUGuest1'/>
       <target dev='hda' bus='ide'/>
       <shareable/>