xend/pvscsi: fix usage of persistant device names for SCSI devices

xend/pvscsi: fix usage of persistant device names for SCSI devices

Currently the callers of vscsi_get_scsidevices() do not pass a mask
string.  This will call "lsscsi -g '[]'", which causes a lsscsi syntax
error. As a result the sysfs parser _vscsi_get_scsidevices() is used.
But this parser is broken and the specified names in the config file are
not found.

Using a mask '*' if no mask was given will call lsscsi correctly and the
following config is parsed correctly:

vscsi=[
	'/dev/sg3, 0:0:0:0',
	'/dev/disk/by-id/wwn-0x600508b4000cf1c30000800000410000, 0:0:0:1'
]

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