.gdbinit: load QEMU sub-commands when gdb starts

The scripts/qemu-gdb.py file is not easily discoverable.  Add a .gdbinit
file so GDB either loads qemu-gdb.py automatically or prints a message
informing the user how to enable them (some systems disable ./.gdbinit
loading for security reasons).

Symlink .gdbinit and the scripts directory in order to make out-of-tree
builds work.  The scripts directory is used to find the qemu-gdb.py file
specified by a relative path in .gdbinit.

Suggested-by: Eric Blake <[email protected]>
Signed-off-by: Stefan Hajnoczi <[email protected]>
Reviewed-by: Alex Bennée <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Tested-by: Eric Blake <[email protected]>
Message-id: 20170517124042[email protected]
Signed-off-by: Stefan Hajnoczi <[email protected]>