doc: Document user creatable object types in help text

doc: Document user creatable object types in help text

The QEMU help for -object is essentially useless, just giving users
the generic syntax. Move it down into its own section and introduce
a nested table where each user creatable object can be documented.
The existing memory-backend-file, rng-random and rng-egd object
types are documented.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>