libxl: support "spice" (remote display protocol) with upstream qemu

libxl: support "spice" (remote display protocol) with upstream qemu

This patch allows you to use spice for
xen-upstream-qemu on upstream Xen or released Xen-4.1.0.

Nothing need to be modified in xen-upstream-qemu,
because qemu has include spice's code as a new feature since qemu-0.14.

Usage:

Add spice fields in VM cfg file.  e.g.
    spice=1
    spiceport=6000
    spicehost='192.168.1.187'
    spicedisable_ticketing = 0 # default is 0
    spicepasswd = 'password'
    spiceagent_mouse = 1 # default is 1

Signed-off-by: Zhou Peng <zhoupeng@nfs.iscas.ac.cn>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>