Duplicated when commit 680d16d added error_set_errno(), and again when
commit 20840d4 added error_set_win32().
Make the original copy in error_set() reusable by factoring out
error_setv(), then rewrite error_set_errno() and error_set_win32() on
top of it.
Backports commit 552375088a832fd5945ede92d01f98977b4eca13 from qemu