The libvirt BlockPull API supports the use of an initial bandwidth limit but the
qemu block_stream API does not.  To get the desired behavior we use the two APIs
strung together: first BlockPull, then BlockJobSetSpeed.  We can do this at the
driver level to avoid duplicated code in each monitor path.
Signed-off-by: NAdam Litke <agl@us.ibm.com>