CA-147743: use blktap3 without kernel grant-copy

This patch series allows blktap3 to be used without having to apply the kernel patch the export the grant-copy functionality to user space. This allows external users of blktap to use the tip instead of some very old version.

tapdisk uses grant-map+memcpy to move data around, which is probably slower than grant-copy, but no slower than blktap2.

Signed-off-by: Thanos Makatos <thanos.makatos@citrix.com>