Adds null check for DisplayStatus (Stefano Stabellini)

Allocate a DisplaySurface in dumb_display_init if none else does it.
The DisplaySurface will be used for the qemu monitor, serial and
parallel ports, etc.

Signed-off-by: Andrew May  <acmay@acmay.homeip.net>
Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>


git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@6387 c046a42c-6fe2-441c-8c8c-71466251a162
