blockjob: wait for pivot to complete

blockjob: wait for pivot to complete

https://bugzilla.redhat.com/show_bug.cgi?id=1119173 documents that
commit eaba79d was flawed in the implementation of the
VIR_DOMAIN_BLOCK_JOB_ABORT_ASYNC flag when it comes to completing
a blockcopy.  Basically, the qemu pivot action is async (the QMP
command returns immediately, but the user must wait for the
BLOCK_JOB_COMPLETE event to know that all I/O related to the job
has finally been flushed), but the libvirt command was documented
as synchronous by default.  As active block commit will also be
using this code, it is worth fixing now.

* src/qemu/qemu_driver.c (qemuDomainBlockJobImpl): Don't skip wait
loop after pivot.

Signed-off-by: Eric Blake <eblake@redhat.com>