Check --store parameter validity before _reserve

Currently the value of the --store parameter is checked after the call
to _reserve is made. Thus the image is actually created on the
database even though the store is invalid, and then returns a E400.

Change-Id: Ic27ed30e605794342161530e8a8eacd6e6deb5d9
Closes-bug: #1260314
Merge "Check --store parameter validity before _reserve"