libxl: prevent creation of domains with duplicate names

libxl: prevent creation of domains with duplicate names

libxl_domain_rename is where domain names are assigned.  Therefore
this is where we check that no two domains have the same name.  As a
special exception, domains whose names are "" are not considered to
clash.

We also take special care not to mind if we try to rename a domain to
the name it already has.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>