vga: Cleanup dirty logging (Jan Kiszka)

vga: Cleanup dirty logging (Jan Kiszka)

In theory, there are no more quirks in the KVM slot management that
requires dirty log start/stop all over the place. We just have to start
the logging each time the mapping may have changed. This patch drops
vga_dirty_log_stop for both standard and cirrus VGA. It also reverts
#6851 as it was obviously a tribute to the old slot system.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@7141 c046a42c-6fe2-441c-8c8c-71466251a162