hw/intc/arm_gicv3_its: Factor out "find address of table entry" code

The ITS has several tables which all share a similar format,
described by the TableDesc struct: the guest may configure them
to be a single-level table or a two-level table. Currently we
open-code the process of finding the table entry in all the
functions which read or write the device table or the collection
table. Factor out the "get the address of the table entry"
logic into a new function, so that the code which needs to
read or write a table entry only needs to call table_entry_addr()
and then perform a suitable load or store to that address.

Note that the error handling is slightly complicated because
we want to handle two cases differently:
 * failure to read the L1 table entry should end up causing
   a command stall, like other kinds of DMA error
 * an L1 table entry that says there is no L2 table for this
   index (ie whose valid bit is 0) must result in us treating
   the table entry as not-valid on read, and discarding
   writes (this is mandated by the spec)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-id: 20220111171048.3545974-12-peter.maydell@linaro.org

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