xend: Device duplicate check fix

xend: Device duplicate check fix

I've checked the duplicate-check code here and I found that's checked
only in the context of one domain but not cross-domain. The thing is
that we should check tap/vbd device cross-domain not to allow another
guest to use the same disk image in some circumstances to prevent VM's
disk corruption.

The patch included denies disk image addition under those
circumstances:

1. We're adding read-only disk that's already used as write-exclusive

2. We're adding write-shared disk that's already used as
write-exclusive

3. We're adding write-exclusive disk that's already used

4. We're adding read-only disk that's already used as write-shared*
(because of I/O caching issues etc.)

The vif device duplicate check remains the same it was and it's
checked in the context of current domain only so that behaviour has
been preserved.

Signed-off-by: Michal Novotny <minovotn@redhat.com>