xl.conf: introduce 'domid_policy'

xl.conf: introduce 'domid_policy'

This patch adds a new global 'domid_policy' configuration option to decide
how domain id values are allocated for new domains. It may be set to one of
two values:

"xen", the default value, will cause an invalid domid value to be passed
to do_domain_create() preserving the existing behaviour of having Xen
choose the domid value during domain_create().

"random" will cause the special RANDOM_DOMID value to be passed to
do_domain_create() such that libxl__domain_make() will select a random
domid value.

Signed-off-by: Paul Durrant <pdurrant@amazon.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>