Fix/cleanup destroyDevice code path in xend.

Fix/cleanup destroyDevice code path in xend.

When calling destroyDevice code path (e.g. xm block-detach dom devid),
allow specifying an integer device id or a device name such as xvdN or
/dev/xvdN.  Allowing the /dev/xvdN form is useful when detaching
devices from dom0.  Bootloaders may do this to unmount a disk
previously mounted in dom0.

Move examination of device ID format into the DevController,
permitting device controllers to determine a valid device ID instead
of higher level code.

Signed-off-by: Jim Fehlig <jfehlig@novell.com>