ring.h: introduce macros to handle monodirectional rings with multiple req sizes

ring.h: introduce macros to handle monodirectional rings with multiple req sizes

This patch introduces macros, structs and functions to handle rings in
the format described by docs/misc/pvcalls.markdown and
docs/misc/9pfs.markdown. The index page (struct __name##_data_intf)
contains the indexes and the grant refs to setup two rings.

               Indexes page
               +----------------------+
               |@0 $NAME_data_intf:   |
               |@76: ring_order = 1   |
               |@80: ref[0]+          |
               |@84: ref[1]+          |
               |           |          |
               |           |          |
               +----------------------+
                           |
                           v (data ring)
                   +-------+-----------+
                   |  @0->4095: in     |
                   |  ref[0]           |
                   |-------------------|
                   |  @4096->8191: out |
                   |  ref[1]           |
                   +-------------------+

$NAME_read_packet and $NAME_write_packet are provided to read or write
any data struct from/to the ring. In pvcalls, they are unused. In xen
9pfs, they are used to read or write the 9pfs header. In other protocols
they could be used to read/write the whole request structure. See
docs/misc/9pfs.markdown:Ring Usage to learn how to check how much data
is on the ring, and how to handle notifications.

There is a ring_size parameter to most functions so that protocols using
these macros don't have to have a statically defined ring order at build
time. In pvcalls for example, each new ring could have a different
order.

These macros don't help you share the indexes page or the event channels
needed for notifications. You can do that with other out of band
mechanisms, such as xenstore or another ring.

It is not possible to use a macro to define another macro with a
variable name. For this reason, this patch introduces static inline
functions instead, that are not C89 compliant. Additionally, the macro
defines a struct with a variable sized array, which is also not C89
compliant.

Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Stefano Stabellini <stefano@aporeto.com>
CC: JBeulich@suse.com