qemu: virtfs: fix wrong variable name

"make QEMU_VIRTFS_ENABLE=y" does not enable VirtFS because of a bad
variable name in common.mk.

Test case:

(host) echo Hello >/tmp/hello
(host) make -j8 run QEMU_VIRTFS_ENABLE=y QEMU_VIRTFS_HOST_DIR=/tmp
(QEMU) mkdir /tmp/host
(QEMU) mount_shared /tmp/host
(QEMU) cat /tmp/host/hello
Hello

Fixes: 97d0529c ("Enable configuring QEMU with Virtfs support")
Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Reviewed-by: Igor Opaniuk <igor.opaniuk@linaro.org>

common.mk

@@ -191,7 +191,7 @@ edk2-clean-common:
 QEMU_CONFIGURE_PARAMS_COMMON = --cc="$(CCACHE)gcc" --extra-cflags="-Wno-error"
 
 ifeq ($(QEMU_VIRTFS_ENABLE),y)
-QEMU_CONFIGURE_PARAMS +=  --enable-virtfs
+QEMU_CONFIGURE_PARAMS_COMMON +=  --enable-virtfs
 QEMU_EXTRA_ARGS +=\
 	-fsdev local,id=fsdev0,path=$(QEMU_VIRTFS_HOST_DIR),security_model=none \
 	-device virtio-9p-device,fsdev=fsdev0,mount_tag=host