osdep: make readv_writev() work with partial read/write

osdep: make readv_writev() work with partial read/write

With a pipe or other reasons, read/write may return less than the
requested bytes. This happens with the test-io-channel-command test on
Windows. glib spawn code uses a binary pipe of 4096 bytes, and the first
read returns that much (although more are requested), for some unclear
reason...

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20221006113657.2656108-2-marcandre.lureau@redhat.com>