libxl: fix memory management in "xl network-attach"

libxl: fix memory management in "xl network-attach"

The libxl_device_nic struct has strings which are initially strdup()ed
and then free()ed in libxl_device_nic_destroy(). In the
"network-attach" parser we need to free() the existing string and
strdup((*argv) + N), rather than just copying the pointer.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>