If someone removes blockcopy storage file when still in mirroring phase
and then requesting blockjob abort using pivot, virsh cmd freezes. This
is not an issue with older qemu versions which did not support
asynchronous jobs (which we prefer by default).
As we have reached the mirroring phase successfully, polling monitor for
blockjob info always returns 1 and the loop never ends.
This fix introduces a check for qemuDomainBlockPivot return code, possibly
skipping the asynchronous waiting completely, if an error occurred and
asynchronous waiting was the preferred method.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1139567