record-btrace: provide xfer_partial target method

record-btrace: provide xfer_partial target method

Provide the xfer_partial target method for the btrace record target.

Only allow memory read accesses to readonly memory while we're replaying,
except for inserting and removing breakpoints.

2014-01-16  Markus Metzger  <markus.t.metzger@intel.com>

	* record-btrace.c (record_btrace_xfer_partial)
	(record_btrace_insert_breakpoint, record_btrace_remove_breakpoint)
	(record_btrace_allow_memory_access): New.
	(init_record_btrace_ops): Initialize new methods.
	* target.c (raw_memory_xfer_partial): Bail out if target reports
	that this memory is not available.