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>

docs/man/xl.conf.5.pod

@@ -45,6 +45,16 @@ The semantics of each C<KEY> defines which form of C<VALUE> is required.
 
 =over 4
 
+=item B<domid_policy="xen"|"random">
+
+Determines how domain-id is set when creating a new domain.
+
+If set to "xen" then the hypervisor will allocate new domain-id values on a sequential basis.
+
+If set to "random" then a random domain-id value will be chosen.
+
+Default: "xen"
+
 =item B<autoballoon="off"|"on"|"auto">
 
 If set to "on" then C<xl> will automatically reduce the amount of