virtfs: allow a device id to be specified in the -virtfs option

When using a virtfs root filesystem, the mount_tag needs to be set to
/dev/root. This can be done long-hand as

  -fsdev local,id=root,path=/path/to/rootfs,...
  -device virtio-9p-pci,fsdev=root,mount_tag=/dev/root

but the -virtfs shortcut cannot be used as it hard-codes the device identifier
to match the mount_tag, and device identifiers may not contain '/':

  $ qemu-system-x86_64 -virtfs local,path=/foo,mount_tag=/dev/root,security_model=passthrough
  qemu-system-x86_64: -virtfs local,path=/foo,mount_tag=/dev/root,security_model=passthrough: duplicate fsdev id: /dev/root

To support this case using -virtfs, we allow the device identifier to be
specified explicitly when the mount_tag is not suitable:

  -virtfs local,id=root,path=/path/to/rootfs,mount_tag=/dev/root,...
Signed-off-by: NChris Webb <chris@arachsys.com>
Signed-off-by: NMichael Tokarev <mjt@tls.msk.ru>

qemu-options.hx

@@ -876,7 +876,7 @@ ETEXI
 
 DEF("virtfs", HAS_ARG, QEMU_OPTION_virtfs,
     "-virtfs local,path=path,mount_tag=tag,security_model=[mapped-xattr|mapped-file|passthrough|none]\n"
-    "        [,writeout=immediate][,readonly][,socket=socket|sock_fd=sock_fd]\n",
+    "        [,id=id][,writeout=immediate][,readonly][,socket=socket|sock_fd=sock_fd]\n",
     QEMU_ARCH_ALL)
 
 STEXI

vl.c

@@ -3524,10 +3524,11 @@ int main(int argc, char **argv, char **envp)
                     exit(1);
                 }
                 fsdev = qemu_opts_create(qemu_find_opts("fsdev"),
+                                         qemu_opts_id(opts) ?:
                                          qemu_opt_get(opts, "mount_tag"),
                                          1, NULL);
                 if (!fsdev) {
-                    error_report("duplicate fsdev id: %s",
+                    error_report("duplicate or invalid fsdev id: %s",
                                  qemu_opt_get(opts, "mount_tag"));
                     exit(1);
                 }
@@ -3565,7 +3566,7 @@ int main(int argc, char **argv, char **envp)
                                           &error_abort);
                 qemu_opt_set(device, "driver", "virtio-9p-pci", &error_abort);
                 qemu_opt_set(device, "fsdev",
-                             qemu_opt_get(opts, "mount_tag"), &error_abort);
+                             qemu_opts_id(fsdev), &error_abort);
                 qemu_opt_set(device, "mount_tag",
                              qemu_opt_get(opts, "mount_tag"), &error_abort);
                 break;