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: NEric Blake <eblake@redhat.com>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: NAlex Bennée <alex.bennee@linaro.org>
Reviewed-by: NPhilippe Mathieu-Daudé <f4bug@amsat.org>
Tested-by: NEric Blake <eblake@redhat.com>
Message-id: 20170517124042.1430-1-stefanha@redhat.com
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>