vring: create a common function to parse descriptors

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
vring: factor common code for error exits

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
dataplane: change vring API to use VirtQueueElement

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
dataplane: replace hostmem with memory_region_find

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>