tests/unit/test-block-iothread: fix maybe-uninitialized error on GCC 11

tests/unit/test-block-iothread: fix maybe-uninitialized error on GCC 11

When building qemu with GCC 11, test-block-iothread produces the following
warning:

../tests/unit/test-block-iothread.c:148:11: error: ‘buf’ may be used
uninitialized [-Werror=maybe-uninitialized]

This is caused by buf[512] left uninitialized and passed to
bdrv_save_vmstate() that expects a const uint8_t *, so the compiler
assumes it will be read and expects the parameter to be initialized.

Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20210319112218.49609-1-eesposit@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>