Fix handling of blkio deviceWeight empty string

Fix handling of blkio deviceWeight empty string

A common coding pattern for changing blkio parameters is

  1. virDomainGetBlkioParameters

  2. change one or more params

  3. virDomainSetBlkioParameters

For this to work, it must be possible to roundtrip through
the methods without error. Unfortunately virDomainGetBlkioParameters
will return "" for the deviceWeight parameter for guests by default,
which virDomainSetBlkioParameters will then reject as invalid.

This fixes the handling of "" to be a no-op, and also improves the
error message to tell you what was invalid