vl: Tighten parsing of -numa's parameter mem

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

    -smp 4 -numa "node,mem=1024,cpus=0-1" -numa "node,mem=1024 cpus=2-3"

are now caught.  Before, the second -numa's argument was silently
interpreted as just "node,mem=1024".

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