When emulating a MOVS from MMIO to guest RAM, the kernel's
vm_copy_setup() wires the destination pages. If the subsequent
MMIO read fails, emulate_movs() skips vm_copy_teardown(), leaking
the page wire references acquired during setup.

Run vm_copy_teardown() regardless of the MMIO read result, and
only copy the value to guest memory if the read succeeds.
Preserve the existing error return.

This matches illumos change 13309.

Obtained from: illumos 83cd75bb2949d26e6eb38ddefc60fdeed1909643
Sponsored by: The FreeBSD Foundation