xl: Fix invalid return of libxl-internal pointers via libxl_domid_to_name()

xl: Fix invalid return of libxl-internal pointers via libxl_domid_to_name()
libxl_domid_to_name has numerous in-and-out-of-library callers. In
library callers now use _libxl_domid_to_name() which participates in
garbage collection and out-of-library callers are fixed up to free() the
domain name.

Signed-off-by: Gianni Tedesco <gianni.tedesco@citrix.com>
Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>