qcow2: Allow -o compat=v3 during qemu-img amend

Commit b76b4f60 allowed '-o compat=v3' as an alias for the
less-appealing '-o compat=1.1' for 'qemu-img create' since we want to
use the QMP form as much as possible, but forgot to do likewise for
qemu-img amend.  Also, it doesn't help that '-o help' doesn't list our
new preferred spellings.
Signed-off-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

block/qcow2.c

@@ -4823,9 +4823,9 @@ static int qcow2_amend_options(BlockDriverState *bs, QemuOpts *opts,
             compat = qemu_opt_get(opts, BLOCK_OPT_COMPAT_LEVEL);
             if (!compat) {
                 /* preserve default */
-            } else if (!strcmp(compat, "0.10")) {
+            } else if (!strcmp(compat, "0.10") || !strcmp(compat, "v2")) {
                 new_version = 2;
-            } else if (!strcmp(compat, "1.1")) {
+            } else if (!strcmp(compat, "1.1") || !strcmp(compat, "v3")) {
                 new_version = 3;
             } else {
                 error_setg(errp, "Unknown compatibility level %s", compat);
@@ -5098,7 +5098,7 @@ static QemuOptsList qcow2_create_opts = {
         {
             .name = BLOCK_OPT_COMPAT_LEVEL,
             .type = QEMU_OPT_STRING,
-            .help = "Compatibility level (0.10 or 1.1)"
+            .help = "Compatibility level (v2 [0.10] or v3 [1.1])"
         },
         {
             .name = BLOCK_OPT_BACKING_FILE,