MI: consider addressable unit size when reading/writing memory

MI: consider addressable unit size when reading/writing memory

As a user of the target memory read/write interface, the MI code must
adjust its memory allocations to take into account the addressable memory
unitsize of the target.

gdb/ChangeLog:

	mi/mi-main.c (mi_cmd_data_read_memory_bytes): Consider byte
	size.
	(mi_cmd_data_write_memory_bytes): Same.