esx: fix memory leak by switching to g_autofree

esx: fix memory leak by switching to g_autofree

volumeName was defined at the top of the function, then a new string
was assigned to it each time through a loop, but after the first
iteration of the loop, the previous string wasn't freed before
allocating a new string the next time. By reducing the scope of
volumeName to be just the loop, and making it g_autofree, we eliminate
the leak.

Signed-off-by: Laine Stump <laine@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>