qemuMigrationSrcNBDStorageCopyReady: Use ready-state of mirror from qemuBlockJobData

qemuMigrationSrcNBDStorageCopyReady: Use ready-state of mirror from qemuBlockJobData

Use the per-job state to determine when the non-shared-storage mirror is
complete rather than the per-disk definition one. The qemuBlockJobData
is a newer approach and is always cleared after the blockjob is
terminated while the 'mirrorState' variable in the definition of the
disk may be left over. In such case the disk mirror would be considered
complete prematurely.

https://bugzilla.redhat.com/show_bug.cgi?id=1889131

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>