xen: optimize xenbus driver for multiple concurrent xenstore accesses

Handling of multiple concurrent Xenstore accesses through xenbus driver
either from the kernel or user land is rather lame today: xenbus is
capable to have one access active only at one point of time.

Rewrite xenbus to handle multiple requests concurrently by making use
of the request id of the Xenstore protocol. This requires to:

- Instead of blocking inside xb_read() when trying to read data from
  the xenstore ring buffer do so only in the main loop of
  xenbus_thread().

- Instead of doing writes to the xenstore ring buffer in the context of
  the caller just queue the request and do the write in the dedicated
  xenbus thread.

- Instead of just forwarding the request id specified by the caller of
  xenbus to xenstore use a xenbus internal unique request id. This will
  allow multiple outstanding requests.

- Modify the locking scheme in order to allow multiple requests being
  active in parallel.

- Instead of waiting for the reply of a user's xenstore request after
  writing the request to the xenstore ring buffer return directly to
  the caller and do the waiting in the read path.

Additionally signal handling was optimized by avoiding waking up the
xenbus thread or sending an event to Xenstore in case the addressed
entity is known to be running already.

As a result communication with Xenstore is sped up by a factor of up
to 5: depending on the request type (read or write) and the amount of
data transferred the gain was at least 20% (small reads) and went up to
a factor of 5 for large writes.

In the end some more rough edges of xenbus have been smoothed:

- Handling of memory shortage when reading from xenstore ring buffer in
  the xenbus driver was not optimal: it was busy looping and issuing a
  warning in each loop.

- In case of xenstore not running in dom0 but in a stubdom we end up
  with two xenbus threads running as the initialization of xenbus in
  dom0 expecting a local xenstored will be redone later when connecting
  to the xenstore domain. Up to now this was no problem as locking
  would prevent the two xenbus threads interfering with each other, but
  this was just a waste of kernel resources.

- An out of memory situation while writing to or reading from the
  xenstore ring buffer no longer will lead to a possible loss of
  synchronization with xenstore.

- The user read and write part are now interruptible by signals.

Signed-off-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>