xend: fix "xm block-detach 0 ..." for extended-ID devices

xend: fix "xm block-detach 0 ..." for  extended-ID devices

Simply taking stat()'s st_rdev doesn't work here, as the minor is
split into two parts, the major is present, and the "extended" bit
isn't set.

Rather than fixing this in a way that would likely be OS-dependent,
simply remove the access to the device file, and instead just parse
the provided string (as is done e.g. for block-attach).

Signed-off-by: Jan Beulich <jbeulich@novell.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>