qemu: add code for handling virtiofsd

Start virtiofsd for each <filesystem> device using it.

Pre-create the socket for communication with QEMU and pass it
to virtiofsd.

Note that virtiofsd needs to run as root.

https://bugzilla.redhat.com/show_bug.cgi?id=1694166

Introduced by QEMU commit a43efa34c7d7b628cbf1ec0fe60043e5c91043ea

Signed-off-by: Ján Tomko <jtomko@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>
Tested-by: Andrea Bolognani <abologna@redhat.com>

tests/qemuxml2argvtest.c

@@ -496,6 +496,17 @@ testCompareXMLToArgv(const void *data)
         }
     }
 
+    for (i = 0; i < vm->def->nfss; i++) {
+        virDomainFSDefPtr fs = vm->def->fss[i];
+        char *s;
+
+        if (fs->fsdriver != VIR_DOMAIN_FS_DRIVER_TYPE_VIRTIOFS)
+            continue;
+
+        s = g_strdup_printf("/tmp/lib/domain--1-guest/fs%zu.vhost-fs.sock", i);
+        QEMU_DOMAIN_FS_PRIVATE(fs)->vhostuser_fs_sock = s;
+    }
+
     if (vm->def->vsock) {
         virDomainVsockDefPtr vsock = vm->def->vsock;
         qemuDomainVsockPrivatePtr vsockPriv =