Add reference to linux flavour usage to virtual targets

All qemu targets miss information regarding login and how to shutdown
the qemu. Add a reference to the section in linux flavour that contains
this information.

Signed-off-by: Luca Seritan <luca.seritan@huawei.com>

docs/hardware-support/virtual-boards/qemuarm-efi.rst

@@ -51,3 +51,5 @@ Once the image is done, you can run the Qemu using the provided script wrapper:
 .. code-block:: console
 
    $ MACHINE=qemuarm-efi runqemu oniro-image-base slirp
+
+For additional information refer to the :ref:`usage section <linux-flavour-usage>`. 

docs/hardware-support/virtual-boards/qemuarm64-efi.rst

@@ -51,3 +51,5 @@ Once the image is done, you can run the Qemu using the provided script wrapper:
 .. code-block:: console
 
    $ MACHINE=qemuarm64-efi runqemu oniro-image-base slirp
+
+For additional information refer to the :ref:`usage section <linux-flavour-usage>`.

docs/hardware-support/virtual-boards/qemux86-64.rst

@@ -49,3 +49,5 @@ Once the image is done, you can run the Qemu using the provided script wrapper:
 .. code-block:: console
 
    $ MACHINE=qemux86-64 runqemu oniro-image-base wic ovmf slirp
+
+For additional information refer to the :ref:`usage section <linux-flavour-usage>`.

docs/hardware-support/virtual-boards/qemux86.rst

@@ -49,3 +49,5 @@ Once the image is done, you can run the Qemu using the provided script wrapper:
 .. code-block:: console
 
    $ MACHINE=qemux86 runqemu oniro-image-base wic ovmf slirp
+
+For additional information refer to the :ref:`usage section <linux-flavour-usage>`.