qemu - How to get higher resolution in xorg in Guest OS

edit the xorg.conf in guest os:

Section "Device"
Identifier "Configured Video Device"
Driver "vesa"
EndSection

Section "Monitor"
Identifier "Configured Monitor"
HorizSync 20.0 - 50.0
VertRefresh 40.0 - 80.0
EndSection

Section "Screen"
Identifier "Default Screen"
Monitor "Configured Monitor"
Device "Configured Video Device"
DefaultDepth 16
Subsection "Display"
Depth 16
Modes "1024x768"
EndSubsection
EndSection