vl: list user creatable properties when 'help' is argument

vl: list user creatable properties when 'help' is argument

Iterate over the writable class properties, sort and print them out
with the description if available.

Ex: qemu -object memory-backend-file,help
memory-backend-file.align=int
memory-backend-file.discard-data=bool
memory-backend-file.dump=bool - Set to 'off' to exclude from core dump
memory-backend-file.host-nodes=int - Binds memory to the list of NUMA host nodes
memory-backend-file.mem-path=string
memory-backend-file.merge=bool - Mark memory as mergeable
memory-backend-file.pmem=bool
memory-backend-file.policy=HostMemPolicy - Set the NUMA policy
memory-backend-file.prealloc=bool - Preallocate memory
memory-backend-file.share=bool - Mark the memory as private to QEMU or shared
memory-backend-file.size=int - Size of the memory region (ex: 500M)

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