win32: add readv/writev emulation

Commit e9d8fbf (qemu-file: do not use stdio for qemu_fdopen, 2013-03-27)
introduced a usage of writev, which mingw32 does not have.  Even though
qemu_fdopen itself is not used on mingw32, the future-proof solution is
to add an implementation of it.  This is simple and similar to how we
emulate sendmsg/recvmsg in util/iov.c.

Some files include osdep.h without qemu-common.h, so move the definition
of iovec to osdep.h too, and include osdep.h from qemu-common.h
unconditionally (protection against including files when NEED_CPU_H is
defined is not needed since the removal of AREG0).

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
