tests/qtest/migration-test: Don't strdup in get_dirty_rate()

tests/qtest/migration-test: Don't strdup in get_dirty_rate()

We g_strdup() the "status" string we get out of the qdict in
get_dirty_rate(), but we never free it.  Since we only use this
string while the dictionary is still valid, we don't need to strdup
at all; drop the unnecessary call to avoid this leak:

Direct leak of 18 byte(s) in 2 object(s) allocated from:
    #0 0x564b3e01913e in malloc (/mnt/nvmedisk/linaro/qemu-from-laptop/qemu/build/asan/tests/qtest/migration-test+0x22f13e) (BuildId: d6403a811332fcc846f93c45e23abfd06d1e67c4)
    #1 0x7f2f278ff738 in g_malloc debian/build/deb/../../../glib/gmem.c:128:13
    #2 0x7f2f27914583 in g_strdup debian/build/deb/../../../glib/gstrfuncs.c:361:17
    #3 0x564b3e14bb5b in get_dirty_rate tests/qtest/migration-test.c:3447:14
    #4 0x564b3e138e00 in test_vcpu_dirty_limit tests/qtest/migration-test.c:3565:16
    #5 0x564b3e14f417 in migration_test_wrapper tests/qtest/migration-helpers.c:456:5

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Fabiano Rosas <farosas@suse.de>
Signed-off-by: Fabiano Rosas <farosas@suse.de>