semihosting: Split out guestfd.c

In arm-compat-semi.c, we have more advanced treatment of
guest file descriptors than we do in other implementations.
Split out GuestFD and related functions to a new file so
that they can be shared.

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