Add grant table support to block tap.

Add grant table support to block tap.

This patch adds grant table support to the block tap.  The AIO support
introduced in patch 9f0eff879d8913a824280cf67658a530c80e8424 still
works -- The tap code maps a granted page twice, once in kernel and
once in user.  The kernel page is patched into the p2m table and pages
added to the user vm_area are mapped to the appropriate underlying
struct pages using the VM_FOREIGN hooks in get_user_pages().

Comparing block IO from dom0 to the existing block backend, and to the
tap managing the same partition as the BE from user space with AIO, I
get the following performance:

Version  1.03       ------Sequential Output------ --Sequential Input- --Random-
                    -Per Chr- --Block-- -Rewrite- -Per Chr- --Block-- --Seeks--
Machine        Size K/sec %CP K/sec %CP K/sec %CP K/sec %CP K/sec %CP  /sec %CP
xen0             2G 31198  95 56818   8 20967   2 28415  77 59595   4 264.9   0
xenU-blkbe2cpuGT 2G 31157  96 54026  10 25585   4 30664  90 64919   7 292.7   0
xenU-blktp2cpuGT 2G 32313  97 54217   8 20950   3 28117  87 65924   4 191.8   0

Signed-off-by: andrew.warfield@cl.cam.ac.uk