hw/9pfs: Add new proxy filesystem driver

Add new proxy filesystem driver to add root privilege to qemu process.
It needs a helper process to be started by root user.

Following command line can be used to utilize proxy filesystem driver
-virtfs proxy,id=<id>,mount_tag=<tag>,socket_fd=<socket-fd>

Signed-off-by: M. Mohan Kumar <mohan@in.ibm.com>
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
diff --git a/vl.c b/vl.c
index 47a8b09..aaa2b93 100644
--- a/vl.c
+++ b/vl.c
@@ -2661,7 +2661,7 @@
             case QEMU_OPTION_virtfs: {
                 QemuOpts *fsdev;
                 QemuOpts *device;
-                const char *writeout;
+                const char *writeout, *sock_fd;
 
                 olist = qemu_find_opts("virtfs");
                 if (!olist) {
@@ -2701,6 +2701,10 @@
                 qemu_opt_set(fsdev, "path", qemu_opt_get(opts, "path"));
                 qemu_opt_set(fsdev, "security_model",
                              qemu_opt_get(opts, "security_model"));
+                sock_fd = qemu_opt_get(opts, "sock_fd");
+                if (sock_fd) {
+                    qemu_opt_set(fsdev, "sock_fd", sock_fd);
+                }
 
                 qemu_opt_set_bool(fsdev, "readonly",
                                 qemu_opt_get_bool(opts, "readonly", 0));