virttest.qemu_vm: add support set VGA vram ram vgamem

For qxl device, we can set global default by params like, qxl_vram_size=67108864 and
qxl_vgamem_mb=16 . For 'std' VGA set vgamem_mb like vga_vgamem_mb=16, 'cirrus'
VGA set like, cirrus-vga_vgamem_mb =16.

Signed-off-by: Xu Tian <xutian@redhat.com>

On 12/19/2014 01:54 PM, CongSmile wrote:
>
> In shared/cfg/base.cfg 
> <https://github.com/autotest/virt-test/pull/1920#discussion-diff-22090809>:
>
> > @@ -255,7 +255,9 @@ boot_menu = off
> >  #### SPICE related options valid if display == spice,
> >  #### you should set vga = qxl to get SPICE in use
> >  #qxl_dev_nr = 1
> > -#qxl_dev_memory = 33554432
> > +#qxl_vgamem_mb = 8
> > +#qxl_vram_size = 33554432
> > +#qxl_ram_size = 33554432
>
> Hi @xutian <https://github.com/xutian>
>
> As those lines are commented, why need add it?
> Or will do it in the internal?
>
Above options  used for qemu older that 0.12,  but we have a request to 
set it in migration testing, so add comments here to give a example for 
guys who want to test it in migration test. And now we set nothing for 
it, by default  qemu use  vgamem_size = 16mb.

Thanks,
Xu
>
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