src: Use virStrcpyStatic() to avoid truncation

src: Use virStrcpyStatic() to avoid truncation

The way virStrncpy() is called here will never result in
buffer overflow, but it won't prevent or detect truncation
either, despite what the error message might suggest. Use
virStrcpyStatic(), which does all of the above, instead.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>