qemu-nbd: allow specifying image as a set of options args

qemu-nbd: allow specifying image as a set of options args

Currently qemu-nbd allows an image filename to be passed on the
command line, but unless using the JSON format, it does not have
a way to set any options except the format eg

   qemu-nbd https://127.0.0.1/images/centos7.iso
   qemu-nbd /home/berrange/demo.qcow2

This adds a --image-opts arg that indicates that the positional
filename should be interpreted as a full option string, not
just a filename.

   qemu-nbd --image-opts driver=https,url=https://127.0.0.1/images,sslverify=off
   qemu-nbd --image-opts driver=file,filename=/home/berrange/demo.qcow2

This flag is mutually exclusive with the '-f' flag.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>