virtio-rng: Add human-readable error message for negative max-bytes parameter

If a negative integer is used for the max_bytes parameter, QEMU currently
calls abort() and leaves behind a core dump. This patch replaces the
abort with a simple error message to make the reason for the termination
clearer. This also ensures device-hotplug with invalid input doesn't
cause qemu to quit.

There is an underlying insufficiency in the parameter parsing code of QEMU
that renders it unable to reject negative values for unsigned properties,
thus the error message "a non-negative integer below 2^63" is the most
user-friendly and correct message we can give until the underlying
insufficiency is corrected.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Amit Shah <amit.shah@redhat.com>
