tools/libxl: Use of init()/dispose() to avoid leaking libxl_dominfo.ssid_label

tools/libxl: Use of init()/dispose() to avoid leaking libxl_dominfo.ssid_label

libxl_dominfo contains a ssid_label pointer which will have memory allocated
for it in libxl_domain_info() if the hypervisor has CONFIG_XSM compiled.
However, the lack of appropriate use of libxl_dominfo_{init,dispose}() will
cause the label string to be leaked, even in success cases.

This was discovered by XenServers Coverity scanning, and are issues not
identified by upstream Coverity Scan.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
CC: Ian Campbell <Ian.Campbell@citrix.com>
CC: Ian Jackson <Ian.Jackson@eu.citrix.com>
CC: Wei Liu <wei.liu2@citrix.com>
CC: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>