Since 4c993d8a we failed to set this important capability, which
allows starting a domain with QXL video card. We set DEVICE_QXL
capability bit instead, which is not necessary wrong. Anyway, if
qemu supports the new '-device qxl' it supports older '-vga qxl'
as well. The latter is used for the primary (the first) qxl video
card, the former for other video cards.