fix xen driver compile issue

[Crazykev]

Ref: #481
Signed-off-by: Crazykev crazykev@zju.edu.cn

[gnawux]

great, thank you @Crazykev

[Crazykev]

@gnawux I introduced this month ago... sorry for that

[gnawux]

I found the building failure when package 0.8, and have not done further investigation on it yet. ref hyperhq/hyperd#591

[Crazykev]

Oh, I see. We did not include xen in CI, #474 will fix this.

	runv  0.8.0
	build OS:   linux-gnu
	prefix:	    /usr/local
	has go:     yes
	with xen:   no
	with libvirt: yes
	has virtualbox: no

[gnawux]

I think neither hykins nor travis support xen hvm mode.