Documentation: teaching: Change QEMU_DISPLAY from sdl to gtk

QEMU_DISPLAY=sdl will not be supported anymore and it doesn't even work for the most recent qemu's versions.
The actual way of opening the console in a GUI is using gtk instead of sdl.
Reference: https://bugs.launchpad.net/ubuntu/+source/qemu/+bug/1829945/comments/1

Author: doringeman

Documentation/teaching/info/vm.rst

@@ -44,7 +44,7 @@ tools/labs:
    char device redirected to /dev/pts/19 (label virtiocon0)
 
 
-.. note:: To show the qemu console use "QEMU_DISPLAY=sdl make
+.. note:: To show the qemu console use "QEMU_DISPLAY=gtk make
           boot". This will show the VGA output and will also give
           access to the standard keyboard.
 

Documentation/teaching/labs/interrupts.rst

@@ -843,7 +843,7 @@ using :command:`dmesg`. Also note that when you use the serial port no
 keyboard interrupt is generated.
 
 .. attention:: To get access to the keyboard on the virtual machine
-	       boot with "QEMU_DISPLAY=sdl make boot".
+	       boot with "QEMU_DISPLAY=gtk make boot".
 
 3. Store ASCII keys to buffer
 -----------------------------

Documentation/teaching/labs/introduction.rst

@@ -526,7 +526,7 @@ a console exposed by the virtual machine using :command:`minicom` or :command:`s
     Poky (Yocto Project Reference Distro) 2.3 qemux86 /dev/hvc0
 
 Alternatively, you can start the virtual machine with graphical interface support, using
-the :command:`QEMU_DISPLAY=sdl make boot`.
+the :command:`QEMU_DISPLAY=gtk make boot`.
 
 .. note::
     To access the virtual machine, at the login prompt, enter the