hw/intc/arm_gicv3_its: Use enum for return value of process_* functions

hw/intc/arm_gicv3_its: Use enum for return value of process_* functions

When an ITS detects an error in a command, it has an
implementation-defined (CONSTRAINED UNPREDICTABLE) choice of whether
to ignore the command, proceeding to the next one in the queue, or to
stall the ITS command queue, processing nothing further.  The
behaviour required when the read of the command packet from memory
fails is less clearly documented, but the same set of choices as for
command errors seem reasonable.

The intention of the QEMU implementation, as documented in the
comments, is that if we encounter a memory error reading the command
packet or one of the various data tables then we should stall, but
for command parameter errors we should ignore the queue and continue.
However, we don't actually do this.  To get the desired behaviour,
the various process_* functions need to return true to cause
process_cmdq() to advance to the next command and keep processing,
and false to stall command processing.  What they mostly do is return
false for any kind of error.

To make the code clearer, replace the 'bool' return from the process_
functions with an enum which may be either CMD_STALL or CMD_CONTINUE.
In this commit no behaviour changes; in subsequent commits we will
adjust the error-return paths for the process_ functions one by one.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20220111171048.3545974-6-peter.maydell@linaro.org