Using proper volume size for volume creating

Correct the logic that used to find the closet capacity to the requested
volume size. Without this change, for example, end user even couldn't
create an 1GB volume from SL backend by jumpgate.

Signed-off-by: Zhi Yan Liu <zhiyanl@cn.ibm.com>
Any reason this try/except shouldn't wrap only the code at lines 317-318, rather than the whole if/else block? Looks like that is the only code here that should raise this exception.