qemu: Restore 9p host mount support

This broke in the qemu-in-images-dir refactor when the config prefix
changed.

board/common/qemu/qemu.sh

@@ -152,9 +152,9 @@ rw_args()
 
 host_args()
 {
-    [ "${QEMU_HOST}" ] || return
+    [ "$CONFIG_QEMU_HOST" ] || return
 
-    echo -n "-virtfs local,path=${QEMU_HOST},security_model=none,writeout=immediate,mount_tag=hostfs "
+    echo -n "-virtfs local,path=$CONFIG_QEMU_HOST,security_model=none,writeout=immediate,mount_tag=hostfs "
 }
 
 net_args()

board/common/rootfs/etc/fstab

@@ -11,9 +11,6 @@ sysfs		/sys		sysfs		defaults		0	0
 debugfs	/sys/kernel/debug	debugfs		nofail			0	0
 cfgfs		/config		configfs	nofail,noauto		0	0
 
-# Optional host share when running in Qemu
-hostfs		/host		9p		nofail,cache=none,msize=16384	0	0
-
 # The chosen backing storage for the overlays placed on /cfg, /etc,
 # /home, /root, and /var, are determined dynamically by /lib/infix/mnt
 # depending on the available devices.