qemu-iotests: step clock after each test iteration

The 093 throttling test submits twice as many requests as the throttle
limit in order to ensure that we reach the limit.  The remaining
requests are left in-flight at the end of each test iteration.

Commit 452589b6b47e8dc6353df257fc803dfc1383bed8 ("vl.c/exit: pause cpus
before closing block devices") exposed a hang in 093.  This happens
because requests are still in flight when QEMU terminates but
QEMU_CLOCK_VIRTUAL time is frozen.  bdrv_drain_all() hangs forever since
throttled requests cannot complete.

Step the clock at the end of each test iteration so in-flight requests
actually finish.  This solves the hang and is cleaner than leaving tests
in-flight.

Note that this could also be "fixed" by disabling throttling when drives
are closed in QEMU.  That approach has two issues:

1. We must drain requests before disabling throttling, so the hang
   cannot be easily avoided!

2. Any time QEMU disables throttling internally there is a chance that
   malicious users can abuse the code path to bypass throttling limits.

Therefore it makes more sense to fix the test case than to modify QEMU.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20170815130502.8736-1-stefanha@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>
