Wire up tapdisk VDI attach, activate, deactivate, detach

When we 'attach' a VDI we create a symlink from the SR/VDI name
to the block device. We can then lookup a Tapctl.tapdev from
this block device. The symlink is stored in a non-persistent
directory so will be cleared on reboot.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>