xm: Some fixes for pvSCSI

xm: Some fixes for pvSCSI

For xm create and xm new, an error may not occur even if wrong
vscsi configuration is given.

e.g.
  vscsi = [ '0:0:0:0,0:0:0:0', '0:0:0:0,1:0:0:0' ]

  # xm create vm1
  Using config file "/etc/xen/vm1".
  Started domain vm1 (id=8)
  # xm scsi-list vm1
  Idx BE  state host  phy-hctl   phy   vir-hctl   devstate
  0   0   1     0     0:0:0:0    sda   0:0:0:0    None
  1   0   1     0     0:0:0:0    sda   1:0:0:0    None

This patch fixes some problems such as the above.

Signed-off-by: Masaki Kanno <kanno.masaki@jp.fujitsu.com>