gdbstub/user-target: fix gdbserver int format (%d -> %x)

gdbstub/user-target: fix gdbserver int format (%d -> %x)

This commit fixes an incorrect format string for formatting integers
provided to GDB when debugging a target run in QEMU user mode.

The correct format is hexadecimal for both success and errno values,
some of which can be seen here [0].

[0] https://github.com/bminor/binutils-gdb/blob/e65a355022d0dc6b5707310876a72b5693ec0aa5/gdbserver/hostio.cc#L196-L213

Signed-off-by: Dominik 'Disconnect3d' Czarnota <dominik.b.czarnota@gmail.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Fixes: e282010b2e1e ("gdbstub: Add support for info proc mappings")
Cc: qemu-stable@nongnu.org
Reviewed-by: Ilya Leoshkevich <iii@linux.ibm.com>
Reviewed-by: Michael Tokarev <mjt@tls.msk.ru>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>