virtio-net: fix use after unmap/free for sg #1202

Job	Run time
build_and_check_fork	0s
build_container	10m 2s
test_installer	54m 11s
sym_trace_tests (x86_64)	40s
taint_tests (i386)	48m 47s
taint_tests (x86_64)	6m 51s
pypanda_tests (dyn_hooks)	14s
make_check (check-qtest-x86_64)	1m 52s
make_check (check-qtest-i386)	1m 51s
make_check (check-qtest-arm)	1m 13s
pypanda_tests (copy_test)	16s
make_check (check-qtest-mips)	36s
pypanda_tests (file_fake)	8s
make_check (check-qtest-mipsel)	37s
pypanda_tests (file_hook)	9s
make_check (check-qtest-ppc)	45s
pypanda_tests (generic_tests)	1m 55s
make_check (check-block)	1m 28s
pypanda_tests (monitor_cmds)	10s
make_check (check-unit)	1m 22s
pypanda_tests (multi_proc_cbs)	21s
make_check (check-qapi-schema)	10s
pypanda_tests (sleep_in_cb)	13s
pypanda_tests (syscalls)	8s
pypanda_tests (record_no_snap)	13s
pypanda_tests (sig_suppress)	21s
cleanup	1s
	2h 14m 34s

Provide feedback