This fixes CONFIG_FB_CIRRUS for Linux guests and probably much more:

When switching away from linearly mapped vram, we also have to restore
the I/O handlers for the LFB.

This regression was once introduced by commit 2bec46dc.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>