util: mdev: Use a local variable instead of a direct pointer access

util: mdev: Use a local variable instead of a direct pointer access

Use a local variable to hold data, rather than accessing the pointer
after calling virMediatedDeviceListAdd (therefore VIR_APPEND_ELEMENT).
Although not causing an issue at the moment, this change is a necessary
prerequisite for tweaking virMediatedDeviceListAdd in a separate patch,
which will take a reference for the source pointer (instead of pointer
value) and will clear it along the way.

(cherry picked from commit 2739a983f235f0d92d210800d61bd1a195f01850)
Signed-off-by: Erik Skultety <eskultet@redhat.com>