qemu-command: use vram attribute for all video devices

qemu-command: use vram attribute for all video devices

So far we didn't have any option to set video memory size for qemu video
devices. There was only the vram (ram for QXL) attribute but it was valid
only for the QXL video device.

To provide this feature to users QEMU has a dedicated device attribute
called 'vgamem_mb' to set the video memory size. We will use the 'vram'
attribute for setting video memory size for other QEMU video devices.

For the cirrus device we will ignore the vram value because it has
hardcoded video size in QEMU.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1076098

Signed-off-by: Pavel Hrdina <phrdina@redhat.com>