virDomainVideoDriverDefParseXML: Allow zero value for @vgaconf
virDomainVideoDriverDefParseXML: Allow zero value for @vgaconf

It was always allowed, but in a very unusual and weird way. Just
look at the original commit that introduced it (78fc843c7b5).
Also, we document that "io" value is accepted (which translates
to VIR_DOMAIN_VIDEO_VGACONF_IO with value of zero).

Signed-off-by: Michal Privoznik <83d82aaba2eed257f4814b0c239c260c4caaadf0@redhat.com>
Reviewed-by: Daniel P. Berrangé <bb938cf255e055ff3507f2627d214e8e62118fcf@redhat.com>

nertpinx/libvirt,libvirt/libvirt,zippy2/libvirt,crobinso/libvirt,nertpinx/libvirt,zippy2/libvirt,libvirt/libvirt,jfehlig/libvirt,olafhering/libvirt,jfehlig/libvirt,nertpinx/libvirt,nertpinx/libvirt,crobinso/libvirt,zippy2/libvirt,libvirt/libvirt,jfehlig/libvirt,zippy2/libvirt,jfehlig/libvirt,nertpinx/libvirt,libvirt/libvirt,olafhering/libvirt,crobinso/libvirt,olafhering/libvirt,olafhering/libvirt,crobinso/libvirt