hv: instr_emul: clean up mmio_read/mmio_write

clean up mmio_read/mmio_write in instr_emul.c
- make these two functions inline
- discard the return value and add the pre condition
- rename these two functions to vie_mmio_read/vie_mmio_write

Tracked-On: #861
Signed-off-by: Shiqing Gao <shiqing.gao@intel.com>
Acked-by: Eddie Dong <eddie.dong@intel.com>

Author: shiqingg