util/cutils: New qemu_strtosz_metric()

To parse numbers with metric suffixes, we use

    qemu_strtosz_suffix_unit(nptr, &eptr, QEMU_STRTOSZ_DEFSUFFIX_B, 1000)

Capture this in a new function for legibility:

    qemu_strtosz_metric(nptr, &eptr)

Replace test_qemu_strtosz_suffix_unit() by test_qemu_strtosz_metric().

Rename qemu_strtosz_suffix_unit() to do_strtosz() and give it internal
linkage.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <1487708048-2131-15-git-send-email-armbru@redhat.com>