iotests: Fix _send_qemu_cmd with bash 5.1

iotests: Fix _send_qemu_cmd with bash 5.1

With bash 5.1, the output of the following script changes:

  a=("double  space")
  a=${a[@]:0:1}
  echo "$a"

from "double space" to "double  space", i.e. all white space is
preserved as-is.  This is probably what we actually want here (judging
from the "...to accommodate pathnames with spaces" comment), but before
5.1, we would have to quote the ${} slice to get the same behavior.

In any case, without quoting, the reference output of many iotests is
different between bash 5.1 and pre-5.1, which is not very good.  The
output of 5.1 is what we want, so whatever we do to get pre-5.1 to the
same result, it means we have to fix the reference output of basically
all tests that invoke _send_qemu_cmd (except the ones that only use
single spaces in the commands they invoke).

Instead of quoting the ${} slice (cmd="${$@: 1:...}"), we can also just
not use array slicing and replace the whole thing with a simple "cmd=$1;
shift", which works because all callers quote the whole $cmd argument
anyway.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20201217153803.101231-3-mreitz@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>