libxc: save/restore error handling fixes

libxc: save/restore error handling fixes

* Make "read_exact" in libxc always set errno.
* Rename "read_exact" macro in xc_domain_restore.c (which shadows
  real function) to RDEXACT and change all callers.
* Make RDEXACT anamorphically use xch for error reporting rather than
* stderr.
* Call PERROR rather than ERROR when appropriate, so that log messages
  include errno.
* Save errno in noncached_write so that its errno value is always
* right.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>