MFC r345956

bhyve: Fix NVMe BAR size calculation

The NVMe specification defines bits 13:4 of BAR0 as Reserved (i.e. 0x0).
Most drivers do not enforce this, but the Windows NVMe driver does and
will refuse to start the device (i.e. error 10) if any of these bits are
set.

Approved by: imp (mentor)