[libvirt] [PATCH]: Fixed minor bugs in display and added OOM checks.

Hi All, There was a minor bug in selecting the graphics type. if the graphics type was desktop it was assumed that display is set for it, and thus crashed on strdup, so now checking if display is present before setting it. The second bug was while setting the 3d acceleration parameter, VIR_ALLOC() returns 0 or greater if success and not the other way round. Lastly added OOM checks in few places which were missed earlier. Regards, Pritesh