In vm_mmap_memseg(), use vm_map_insert() instead of vm_map_find().
Existing callers expect to map the GPA that they passed, whereas
vm_map_find() merely treats the GPA as a hint. Also check for overflow
and remove a test for first < 0 since "first" is unsigned.

In vmm_mmio_alloc(), return an error number instead of an object
pointer, since the sole caller doesn't need the pointer. As in
vm_mmap_memseg(), use vm_map_insert() instead of vm_map_find() and
validate parameters. This function is not directly reachable via
ioctl(), but we ought to be careful anyway.