target/arm/arm-semi: fix SYS_OPEN to return nonzero filehandle

According to the specification "Semihosting for AArch32 and Aarch64",
the SYS_OPEN operation should return:

- A nonzero handle if the call is successful
- -1 if the call is not successful

So, it should never return 0.

Prior to commit 35e9a0a8ce4b ("target/arm/arm-semi: Make semihosting
code hand out its own file descriptors"), the guest fd matched to the
host fd. It returned a nonzero handle on success since the fd 0 is
already used for stdin.

Now that the guest fd is the index of guestfd_array, it starts from 0.

I noticed this issue particularly because Trusted Firmware-A built with
PLAT=qemu is no longer working. Its io_semihosting driver only handles
a positive return value as a valid filehandle.

Basically, there are two ways to fix this:

- Use (guestfd - 1) as the index of guestfs_arrary. We need to insert
increment/decrement to convert the guestfd and the array index back
and forth.

- Keep using guestfd as the index of guestfs_array. The first entry
of guestfs_array is left unused.

I thought the latter is simpler. We end up with wasting a small piece
of memory for the unused first entry of guestfd_array, but this is
probably not a big deal.

Fixes: 35e9a0a8ce4b ("target/arm/arm-semi: Make semihosting code hand out its own file descriptors")
Cc: qemu-stable@nongnu.org
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20200109041228.10131-1-masahiroy@kernel.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>