libxl: Offer API versions 0x040700 and 0x040800

libxl: Offer API versions 0x040700 and 0x040800

According to git log -G:

0x040700 was introduced in 304400459ef0 (aka 4.7.0-rc1~481)
  "tools/libxl: rename remus device to checkpoint device"

0x040800 was introduced in 57f8b13c7240 (aka 4.8.0-rc1~437)
  "libxl: memory size in kb requires 64 bit variable"

It is surprising that no-one noticed this.

Anyway, in the meantime, we should fix it.  Backporting this is
probably a good idea: it won't change the behaviour for existing
callers but it will avoid errors for some older correct uses.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Anthony PERARD <anthony.perard@citrix.com>
Release-acked-by: Juergen Gross <jgross@suse.com>