storage: Fix lvcreate parameter for backingStore.

storage: Fix lvcreate parameter for backingStore.

When virStorageBackendLogicalCreateVol() creates a snapshot for a
logical volume with backingStore element, it fails with the message
below:

  2013-01-17 03:10:18.869+0000: 1967: error : virCommandWait:2345 :
  internal error Child process (/sbin/lvcreate --name lvm-snapshot -L 51200K
  -s=/dev/lvm-pool/lvm-volume) unexpected exit status 3: /sbin/lvcreate:
  invalid option -- '='  Error during parsing of command line.

This is because virCommandAddArgPair() uses '=' to connect the two
parameters, it's unsuitable for -s option of the lvcreate.

Signed-off-by: Atsushi Kumagai <kumagai-atsushi@mxc.nes.nec.co.jp>