qemu: Update state of block job to READY only if it actually is ready

qemu: Update state of block job to READY only if it actually is ready

Few parts of the code looked at the current progress of and assumed that
a two phase blockjob is in the _READY state as soon as the progress
reached 100% (info.cur == info.end). In current versions of qemu this
assumption is invalid and qemu exposes a new flag 'ready' in the
query-block-jobs output that is set to true if the job is actually
finished.

This patch adds internal data handling for reading the 'ready' flag and
acting appropriately as long as the flag is present.

While this still doesn't fix the virsh client problem with two phase
block jobs and the --pivot option, it at least improves the error
message:

$ virsh blockcommit  --wait --verbose vm vda  --base vda[1] --active --pivot
Block commit: [100 %]error: failed to pivot job for disk vda
error: internal error: unable to execute QEMU command 'block-job-complete': The active block job for device 'drive-virtio-disk0' cannot be completed

to

$ virsh blockcommit  --wait --verbose VM vda  --base vda[1] --active --pivot
Block commit: [100 %]error: failed to pivot job for disk vda
error: block copy still active: disk 'vda' not ready for pivot yet