passthrough: fix writing handlers for base address registers.

passthrough: fix writing handlers for base address registers.

- Current implementation can not work fine when base address registers
  are accessed via 1 byte write access and 2 byte write access. This
  patch enables them.

- Currently guest software can set address which is not aligned
  with resource size and page size. The patch does not allow guest
  software to set unaligned address.

Signed-off-by: Yuji Shimada <shimada-yxb@necst.nec.co.jp>