libxl: don't leak gc pointers to caller's structs; prevent double free

libxl: don't leak gc pointers to caller's structs; prevent double free

libxl_build_device_model uses a pointer in a caller supplied data
structure to synthesize a vif-name if one is not supplied. This is bad
juju because the caller may want to free this pointer but by the time it
get's a chance the gc has already done so. Switch to using a local
variable for this pointer and avoid a double-free in the domain create
path.

Gianni Tedesco <gianni.tedesco@citrix.com>
Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>