virsh: improve the error for wrong memtune parameters

virsh: improve the error for wrong memtune parameters

When set guest memory with a invalid parameter of --soft-limit,
it posts weird error:

$ virsh memtune r7 --hard-limit 20417224 --soft-limit 9007199254740992 \
  --swap-hard-limit 35417224
error: Unable to parse integer parameter 'NAME

Change it to

error: Unable to parse integer parameter soft-limit

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1211550
Signed-off-by: Shanzhi Yu <shyu@redhat.com>
Signed-off-by: Ján Tomko <jtomko@redhat.com>