qemu: add .exrc

I've been using this to get correct indenting with vim
in qemu for a while, but it's a bit easier if we
put the settings in the central place.
Note that
1. you still need to enable 'exrc' and 'secure'
   options in your vimrc for these settings to take effect.
2. you can create a .vimrc file if 'exrc' is on but there's
   need to bypass this configuration.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
