Fix virFileReadLimFD/virFileReadAll to handle EINTR

Fix virFileReadLimFD/virFileReadAll to handle EINTR

The fread_file_lim() function uses fread() but never handles
EINTR results, causing unexpected failures when reading QEMU
help arg info. It was unneccessarily using FILE * instead
of plain UNIX file handles, which prevented use of saferead()

* src/util/util.c: Switch fread_file_lim over to use saferead
  instead of fread, remove FILE * use, and rename