gdbstub: rename cpu_index -> cpu_gdb_index

gdbstub: rename cpu_index -> cpu_gdb_index

This is to make it clear the index is purely a gdbstub function and
should not be confused with the value of cpu->cpu_index. At the same
time we move the function from the header to gdbstub itself which will
help with later changes.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Claudio Imbrenda <imbrenda@linux.vnet.ibm.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20170712105216.747-3-alex.bennee@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>