vl: Tighten parsing of -m argument

strtosz_suffix() fails unless the size is followed by 0, whitespace or
','.  Useless here, because we need to fail for any junk following the
size, even if it starts with whitespace or ','.  Check manually.
Things like "-m 1024," are now caught.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
