hw/core/loader.c: Track last-seen ROM in rom_check_and_register_reset()

hw/core/loader.c: Track last-seen ROM in rom_check_and_register_reset()

In rom_check_and_register_reset() we detect overlaps by looking at
whether the ROM blob we're currently examining is in the same address
space and starts before the previous ROM blob ends.  (This works
because the ROM list is kept sorted in order by AddressSpace and then
by address.)

Instead of keeping the AddressSpace and last address of the previous ROM
blob in local variables, just keep a pointer to it.

This will allow us to print more useful information when we do detect
an overlap.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20201129203923.10622-2-peter.maydell@linaro.org