Fix a memory leak in the snapshot code in libvirtd.

Fix a memory leak in the snapshot code in libvirtd.

While running libvirtd under valgrind and doing some
snapshot testing I noticed that we would always leak a
connection reference.  The problem was actually that we
were leaking a domain reference in the libvirtd remote
snapshot code, which was in turn causing a leaked
connection reference.  Fix the situation by explicitly
taking and dropping a domain reference where we need it.

Signed-off-by: Chris Lalancette <clalance@redhat.com>