virsh: fix keepalive error msg
resolves https://bugzilla.redhat.com/show_bug.cgi?id=1132305:
The error message for an out-of-range argument was confusing:
virsh -k 9999999999
error: option --k requires a positive numeric argument
After this patch, it is:
error: Invalid value for option -k
Signed-off-by: Eric Blake <eblake@redhat.com>