virDomainBlockCopy: Introduce VIR_DOMAIN_BLOCK_COPY_SYNCHRONOUS_WRITES flag

virDomainBlockCopy: Introduce VIR_DOMAIN_BLOCK_COPY_SYNCHRONOUS_WRITES flag

In cases when the destination storage is slower than the normal VM
storage and the VM does intensive I/O to the disk a block copy job may
never converge.

Switching it to synchronous mode will ensure that all writes done by the
guest are propagated to the destination at the cost of slowing down I/O
of the guest to the synchronous speed.

This patch adds the new API flag and implements virsh support.

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