vhost-user: support cross-endian vnet headers

vhost-user: support cross-endian vnet headers

vhost-user already has a way to communicate the endianness of the guest
via the vring endianness messages.  The vring endianness always matches
the vnet header endianness so there is no need to do anything else in
the backend.

Reviewed-by: Marc-André Lureau <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>
Message-Id: <1543851204[email protected]>
Message-Id: <1550165756[email protected]>
Reviewed-by: Michael S. Tsirkin <[email protected]>
Signed-off-by: Michael S. Tsirkin <[email protected]>