lib/alloc: simplify free and malloc

Remove the size parameter from the various free functions

Since the backends can handle the allocation sizes on their own,
simplify the generic malloc wrappers.

Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Message-Id: <20201002154420.292134-6-imbrenda@linux.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>