block/iscsi: Use lock guard macros

Replace manual lock()/unlock() calls with lock guard macros
(QEMU_LOCK_GUARD/WITH_QEMU_LOCK_GUARD) in block/iscsi.

Signed-off-by: Gan Qixin <ganqixin@huawei.com>
Message-Id: <20201203075055.127773-5-ganqixin@huawei.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>