storage: Need to properly read the crypt offset value

storage: Need to properly read the crypt offset value

Commit id 'a48c7141' altered how to determine if a volume was encrypted
by adding a peek at an offset into the file at a specific buffer location.
Unfortunately, all that was compared was the first "char" of the buffer
against the expect "int" value.

Restore the virReadBufInt32BE to get the complete field in order to
compare against the expected value from the qcow2EncryptionInfo or
qcow1EncryptionInfo "modeValue" field.

This restores the capability to create a volume with encryption, then
refresh the pool, and still find the encryption for the volume.