qtest/libqtest: fix heap-buffer-overflow in qtest_cb_for_every_machine()

When the length of mname is less than 5, memcpy("xenfv", mname, 5) will cause
heap buffer overflow. Therefore, use strncmp to avoid this problem.

The asan showed stack:

ERROR: AddressSanitizer: heap-buffer-overflow on address 0x60200000f2f4 at
pc 0x7f65d8cc2225 bp 0x7ffe93cc5a60 sp 0x7ffe93cc5208 READ of size 5 at
0x60200000f2f4 thread T0
    #0 0x7f65d8cc2224 in memcmp (/lib64/libasan.so.5+0xdf224)
    #1 0x5632c20be95b in qtest_cb_for_every_machine tests/qtest/libqtest.c:1282
    #2 0x5632c20b7995 in main tests/qtest/test-hmp.c:160
    #3 0x7f65d88fed42 in __libc_start_main (/lib64/libc.so.6+0x26d42)
    #4 0x5632c20b72cd in _start (build/tests/qtest/test-hmp+0x542cd)

Reported-by: Euler Robot <euler.robot@huawei.com>
Signed-off-by: Gan Qixin <ganqixin@huawei.com>
Reviewed-by: Laurent Vivier <lvivier@redhat.com>
Message-Id: <20210106050625.518041-1-ganqixin@huawei.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>