xen, blkfront: introduce support for multiple block rings

This commit introduces in xen-blkfront actual support for multiple
block rings. The number of block rings to be used is still forced
to one.

Signed-off-by: Arianna Avanzini <avanzini.arianna@gmail.com>
xen, blkfront: negotiate the number of block rings with the backend

This commit implements the negotiation of the number of block rings
to be used; as a default, the number of rings is decided by the
frontend driver and is equal to the number of hardware queues that
the backend makes available. In case of guest migration towards a
host whose devices expose a different number of hardware queues, the
number of I/O rings used by the frontend driver remains the same;
XenStore keys may vary if the frontend needs to be compatible with
a host not having multi-queue support.

Signed-off-by: Arianna Avanzini <avanzini.arianna@gmail.com>
xen, blkback: introduce support for multiple block rings

This commit adds to xen-blkback the support to map and make use
of a variable number of ringbuffers. The number of rings to be
mapped is forcedly set to one.

Signed-off-by: Arianna Avanzini <avanzini.arianna@gmail.com>
xen, blkback: negotiate of the number of block rings with the frontend

This commit lets the backend driver advertise the number of available
hardware queues; it also implements gathering from the frontend driver
the number of rings actually available for mapping.

Signed-off-by: Arianna Avanzini <avanzini.arianna@gmail.com>