Support for multiple heads for Qxl display device

Bug-Url: https://bugzilla.redhat.com/787578
Change-Id: I0799193df70edf7e0be2c172636df229c59d6aac
Signed-off-by: Shahar Havivi <shaharh@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/17067
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Tested-by: Shahar Havivi <shavivi@redhat.com>