xend: A few blktap2 fixes

xend: A few blktap2 fixes

1. Bug fix for error: "Error: Device /dev/xvdp (51952, tap2) is
already connected." (xenstore does not clean after DomU stoped)
2. Bug fix for error: "File 'vhd:/path/.../disk.img' doesn't exist."
(not correct parsing)
3. Bug fix for error: "Error: Device 51952 not connected" (in config
file for DomU we should be use prefix "tap2:tapdisk:xxx" for devices
from (aio, ram, qcow, vhd, remus) or "tap:tapdisk:xxx" for devices
from (sync, vmdk, qcow2, ioemu))
4. Bug fix for error: "Disk is not accessible" (if use 'tap2'-device
type, then '/dev/xpvd' may not be accessible immediately after its
creation)

Signed-off-by: eXeC001er <execooler@gmail.com>