consolidate qemu_iovec_memset{,_skip}() into single function and use existing iov_memset()

This patch combines two functions into one, and replaces
the implementation with already existing iov_memset() from
iov.c.

The new prototype of qemu_iovec_memset():
  size_t qemu_iovec_memset(qiov, size_t offset, int fillc, size_t bytes)
It is different from former qemu_iovec_memset_skip(), and
I want to make other functions to be consistent with it
too: first how much to skip, second what, and 3rd how many
of it.  It also returns actual number of bytes filled in,
which may be less than the requested `bytes' if qiov is
smaller than offset+bytes, in the same way iov_memset()
does.

While at it, use utility function iov_memset() from
iov.h in posix-aio-compat.c, where qiov was used.

Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
diff --git a/qemu-common.h b/qemu-common.h
index 91e0562..e752d2b 100644
--- a/qemu-common.h
+++ b/qemu-common.h
@@ -347,9 +347,8 @@
 void qemu_iovec_reset(QEMUIOVector *qiov);
 void qemu_iovec_to_buffer(QEMUIOVector *qiov, void *buf);
 void qemu_iovec_from_buffer(QEMUIOVector *qiov, const void *buf, size_t count);
-void qemu_iovec_memset(QEMUIOVector *qiov, int c, size_t count);
-void qemu_iovec_memset_skip(QEMUIOVector *qiov, int c, size_t count,
-                            size_t skip);
+size_t qemu_iovec_memset(QEMUIOVector *qiov, size_t offset,
+                         int fillc, size_t bytes);
 
 bool buffer_is_zero(const void *buf, size_t len);