migration/dirtyrate: Silence warning about strcpy() on OpenBSD

migration/dirtyrate: Silence warning about strcpy() on OpenBSD

The linker on OpenBSD complains:

 ld: warning: dirtyrate.c:447 (../src/migration/dirtyrate.c:447)(...):
 warning: strcpy() is almost always misused, please use strlcpy()

It's currently not a real problem in this case since both arrays
have the same size (256 bytes). But just in case somebody changes
the size of the source array in the future, let's better play safe
and use g_strlcpy() here instead, with an additional check that the
string has been copied as a whole.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Hyman Huang <yong.huang@smartx.com>
Link: https://lore.kernel.org/r/20241022063402.184213-1-thuth@redhat.com
[peterx: Fix over-80 chars]
Signed-off-by: Peter Xu <peterx@redhat.com>