xend/pvscsi: update sysfs parser for Linux 3.0

xend/pvscsi: update sysfs parser for Linux 3.0

The sysfs parser for /sys/bus/scsi/devices understands only the layout
of kernel version 2.6.16. This looks as follows:

/sys/bus/scsi/devices/1:0:0:0/block:sda is a symlink to /sys/block/sda/
/sys/bus/scsi/devices/1:0:0:0/scsi_generic:sg1 is a symlink to /sys/class/scsi_generic/sg1

Both directories contain a 'dev' file with the major:minor information.
This patch updates the used regex strings to match also the colon to
make it more robust against possible future changes.

In kernel version 3.0 the layout changed:
/sys/bus/scsi/devices/ contains now additional symlinks to directories
such as host1 and target1:0:0. This patch ignores these as they do not
point to the desired scsi devices. They just clutter the devices array.

The directory layout in '1:0:0:0' changed as well, the 'type:name'
notation was replaced with 'type/name' directories:

/sys/bus/scsi/devices/1:0:0:0/block/sda/
/sys/bus/scsi/devices/1:0:0:0/scsi_generic/sg1/

Both directories contain a 'dev' file with the major:minor information.
This patch adds additional code to walk the subdir to find the 'dev'
file to make sure the given subdirectory is really the kernel name.

In addition this patch makes sure devname is not None.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Committed-by: Ian Campbell <ian.campbell@citrix.com>