Linux from around 5.6 now enumerates individual disks in any order
(whereas previously it enumerated only drivers in parallel). This
means that /dev/sdX ordering is no longer stable - in particular we
cannot be sure that /dev/sda inside the guest is the first disk that
was attached to the appliance, /dev/sdb the second disk and so on.
However we can still use SCSI PCI device numbering as found in
/dev/disk/by-path. Use this to translate device names in and out of
the appliance.
Thanks: Vitaly Kuznetsov, Paolo Bonzini, Dan Berrangé.
This function doesn't work reliably with the proposed change to device
name translation. The reason is that strings returned by
Devsparts.list_devices contained translated names, so their indexes
did not correspond to the untranslated names used outside the
appliance..
We can avoid this and make the function much simpler and faster by
implementing it on the library side instead.
This safe wrapper around Unix.openfile ensures that exceptions
escaping cannot leave unclosed files.
There are only a few places in the code where this wrapper can be used
currently. There are other occurences of Unix.openfile but they are
not suitable for replacement.
