Support xl as a xen interface command

xm and xl are mostly compatible, there is just a change in migrate,
which for now we simple bridge by comparing the command string.

Also XEN_CMD is now set during configure time. This will then be fixed
in Ganeti 2.7 when we make it an hvparams.

Signed-off-by: René Nussbaumer <rn@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>