virsh: avoid strncpy

virsh: avoid strncpy

strncpy is generally evil - it runs the risk of missing NUL
termination, and more often than not wastes time zeroing way
more bytes than strictly necessary.  We've avoided this evil
in our virStrncpy wrapper, except for places where we forgot
to use the wrapper; meanwhile, we have also added an even
higher layer wrapper for setting virTypedParameter values.

* tools/virsh.c (cmdMemtune, cmdBlkdeviotune): Use modern API.
* cfg.mk (exclude_file_name_regexp--sc_prohibit_strncpy): Tighten.