hw/intc/arm_gicv3_its: Zero initialize local DTEntry etc structs

hw/intc/arm_gicv3_its: Zero initialize local DTEntry etc structs

In the GICv3 ITS model, we have a common coding pattern which has a
local C struct like "DTEntry dte", which is a C representation of an
in-guest-memory data structure, and we call a function such as
get_dte() to read guest memory and fill in the C struct.  These
functions to read in the struct sometimes have cases where they will
leave early and not fill in the whole struct (for instance get_dte()
will set "dte->valid = false" and nothing else for the case where it
is passed an entry_addr implying that there is no L2 table entry for
the DTE).  This then causes potential use of uninitialized memory
later, for instance when we call a trace event which prints all the
fields of the struct.  Sufficiently advanced compilers may produce
-Wmaybe-uninitialized warnings about this, especially if LTO is
enabled.

Rather than trying to carefully separate out these trace events into
"only the 'valid' field is initialized" and "all fields can be
printed", zero-init all the structs when we define them. None of
these structs are large (the biggest is 24 bytes) and having
consistent behaviour is less likely to be buggy.

Cc: qemu-stable@nongnu.org
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2718
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-id: 20241213182337.3343068-1-peter.maydell@linaro.org
(cherry picked from commit 9678b9c505725732353baefedb88b53c2eb8a184)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>