linux-stubdom: use "vga" aka "stdvga" video card
linux-stubdom: use "vga" aka "stdvga" video card

In contrast to the cirrus card emulation the vga card supports more
video modes. Since the qemu gui-agent bug [1] is now fixed we can switch
back to use it.

[1]: QubesOS/qubes-linux-utils@f14637a61557b075ca34424135c90dc2476d67d7

woju/qubes-core-admin,QubesOS/qubes-core-admin,marmarek/qubes-core-admin,marmarek/qubes-core-admin,marmarek/qubes-core-admin,QubesOS/qubes-core-admin,QubesOS/qubes-core-admin,woju/qubes-core-admin,woju/qubes-core-admin,woju/qubes-core-admin