Adjust read_value_memory to use to_xfer_partial

Adjust read_value_memory to use to_xfer_partial

As the new to_xfer_partial implementations are done in ctf and tfile
targets, read_value_memory can be simplified a lot.  Call
target_xfer_partial in a loop, check return value, and set bytes
unavailable when necessary.

gdb:

2014-02-23  Yao Qi  <yao@codesourcery.com>

	* valops.c (read_value_memory): Rewrite it.  Call
	target_xfer_partial in a loop.
	* exec.h (section_table_available_memory): Remove declaration.
	Move comments to ...
	* exec.c (section_table_available_memory): ... here.  Make it static.