hw/ivshmem.c don't check for negative values on unsigned data types

There is no need to check for dest < 0 or vector >= 0 as both are
uint16_t.

This should fix problems with broken build with aggressive compiler
flags. Reported by Xudong Hao <xudong.hao@intel.com>

Signed-off-by: Jes Sorensen <Jes.Sorensen@redhat.com>
Acked-by: Cam Macdonell <cam@cs.ualberta.ca>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
