libxl: rename stubdomain when renaming domain.

libxl: rename stubdomain when renaming domain.

Otherwise after a localhost migrate of an HVM domain with a stubdomain we end
up with domains called "FOO" and "FOO--incoming-dm". This because we initially
create the domains as "FOO--incoming" and then rename to "FOO" inorder to
maintain the uniqueness of domain names on a host.

In this state a second attempt to migrate will fail upon attempting to create
a new domain named "FOO--incoming-dm"

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Committed-by: Ian Campbell <ian.campbell@citrix.com>