qemu.tests: new pxe boot tests

check vm not hung when query cpus during pxe boot

Signed-off-by: Xu Tian <xutian@redhat.com>
virt.tests: add configuration for pxe boot with query cpus tests

Signed-off-by: Xu Tian <xutian@redhat.com>
virt.tests: update pxe test steps info

Signed-off-by: Xu Tian <xutian@redhat.com>