Allow to enlarge or shrink the screen via CTRL-ALT-+/-. In contrast to
scaling the window, these controls always preserve the aspect ratio of
the current console.

CC: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>