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 04:00 PM, CongSmile wrote:
>
> In virttest/qemu_vm.py 
> <https://github.com/autotest/virt-test/pull/1920#discussion-diff-22093455>:
>
> > @@ -1170,14 +1165,15 @@ def add_balloon(devices, devid=None, bus=None, use_old_format=None):
> >                  devices.insert(StrDev('VGA-%s' % vga,
> >                                        cmdline=add_vga(vga),
> >                                        parent_bus={'aobject': 'pci.0'}))
> > +                if vga == 'qxl':
> > +                    qxl_dev_nr = int(params.get("qxl_dev_nr", 1))
> > +                    if qxl_dev_nr > 1:
> > +                        addr = int(params.get("qxl_base_addr", 29))
> > +                        cmdline = add_qxl(qxl_dev_nr, addr)
> > +                        devices.insert(StrDev('qxl', cmdline=cmdline))
>
> Hi @xutian <https://github.com/xutian>
>
> One more question.
>
> Should be a for / while loop here, right?
>
> If qxl_dev_nr =5, we need add each device every time.
> We cant add it in one time.
>
> Am I misunderstanding?
>
This part keep original implementation,  number of qxl device handle in 
add_qxl function.

Thanks,
Xu
>
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> <https://github.com/autotest/virt-test/pull/1920/files#r22093455>.
>