# Create image, 10G in size, qcow2 format
qemu-img create -f qcow2 debian.img 10G

# To launch from cdrom, add '-cdrom /path/to/iso/image.iso -boot order=dc' to
# qemu-system-x86_64 cmdline

# Launch qemu with gdb listening on port 8888, monitor on 8889
qemu-system-x86_64 -m 1G  -gdb tcp:127.0.0.1:8888 -monitor telnet:127.0.0.1:8889,server,nowait -enable-kvm debian.img

# then you can connect to it in gdb using 'target remote localhost:8888'