qemu-option: improve qemu_opts_print_help() output

qemu-option: improve qemu_opts_print_help() output

Modify qemu_opts_print_help():
- to print expected argument type
- skip description if not available
- sort lines
- prefix with the list name (like qdev, to avoid confusion)
- drop 16-chars alignment, use a '-' as seperator for option name and
  description

For ex, "-spice help" output is changed from:

port             No description available
tls-port         No description available
addr             No description available
[...]
gl               No description available
rendernode       No description available

to:

spice.addr=str
spice.agent-mouse=bool (on/off)
spice.disable-agent-file-xfer=bool (on/off)
[...]
spice.x509-key-password=str
spice.zlib-glz-wan-compression=str

"qemu-img create -f qcow2 -o help", changed from:

size             Virtual disk size
compat           Compatibility level (0.10 or 1.1)
backing_file     File name of a base image
[...]
lazy_refcounts   Postpone refcount updates
refcount_bits    Width of a reference count entry in bits

to:

backing_file=str - File name of a base image
backing_fmt=str - Image format of the base image
cluster_size=size - qcow2 cluster size
[...]
refcount_bits=num - Width of a reference count entry in bits
size=size - Virtual disk size

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>