vl.c: Fix memory leak in qemu_register_machine()

Since commit 261747f176f6 (vl: Use MachineClass instead of global
QEMUMachine list) valgrind complains about the following:

==54082== 57 bytes in 3 blocks are definitely lost in loss record 365 of
729
==54082==    at 0x4031AFE: malloc (vg_replace_malloc.c:292)
==54082==    by 0x4145569: g_malloc (in
/usr/lib64/libglib-2.0.so.0.3400.2)
==54082==    by 0x415F9E9: g_strconcat (in
/usr/lib64/libglib-2.0.so.0.3400.2)
==54082==    by 0x80157FE7: qemu_register_machine (vl.c:1597)
==54082==    by 0x80208E6B: module_call_init (module.c:105)
==54082==    by 0x80013B91: main (vl.c:3000)

Turns out that valgrind is right. We simply forget the memory that
g_strconcat() has allocated. Lets free it after the type_register().
We need a 2nd variable due to constness of the name part of the
type structure.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>
