qemu: blockCopy: Pass adjusted bandwidth when called via blockRebase
The block copy API takes the speed in bytes/s rather than MiB/s that was
the prior approach in virDomainBlockRebase. We correctly converted the
speed to bytes/s in the old API but we still called the common helper
virDomainBlockCopyCommon with the unadjusted variable.
Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1207122