blockjob: hoist bandwidth scaling out of monitor code

blockjob: hoist bandwidth scaling out of monitor code

qemu treats blockjob bandwidth as a 64-bit number, in the units
of bytes/second.  But we stupidly modeled block job bandwidth
after migration bandwidth, which in turn was an 'unsigned long'
and therefore subject to 32-bit vs. 64-bit interpretations, and
with a scale of MiB/s.  Our code already has to convert between
the two scales, and report overflow as appropriate; although
this conversion currently lives in the monitor code.  In fact,
our conversion code limited things to 63 bits, because we
checked against LLONG_MAX and reject what would be negative
bandwidth if treated as signed.

On the bright side, our use of MiB/s means that even with a
32-bit unsigned long, we still have no problem representing a
bandwidth of 2GiB/s, which is starting to be more feasible as
10-gigabit or even faster interfaces are used.  And once you
get past the physical speeds of existing interfaces, any larger
bandwidth number behaves the same - effectively unlimited.
But on the low side, the granularity of 1MiB/s tuning is rather
coarse.  So the new virDomainBlockJob API decided to go with
a direct 64-bit bytes/sec number instead of the scaled number
that prior blockjob APIs had used.  But there is no point in
rounding this number to MiB/s just to scale it back to bytes/s
for handing to qemu.

In order to make future code sharing possible between the old
virDomainBlockRebase and the new virDomainBlockCopy, this patch
moves the scaling and overflow detection into the driver code.
Several of the block job calls that can set speed are fed
through a common interface, so it was easier to adjust all block
jobs at once, for consistency.  This patch is just code motion;
there should be no user-visible change in behavior.

* src/qemu/qemu_monitor.h (qemuMonitorBlockJob)
(qemuMonitorBlockCommit, qemuMonitorDriveMirror): Change
parameter type and scale.
* src/qemu/qemu_monitor.c (qemuMonitorBlockJob)
(qemuMonitorBlockCommit, qemuMonitorDriveMirror): Move scaling
and overflow detection...
* src/qemu/qemu_driver.c (qemuDomainBlockJobImpl)
(qemuDomainBlockRebase, qemuDomainBlockCommit): ...here.
(qemuDomainBlockCopy): Use bytes/sec.

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