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/Makefile b/Makefile
index 9b7a85e..017836b 100644
--- a/Makefile
+++ b/Makefile
@@ -154,7 +154,8 @@
 qemu-img.o qemu-tool.o qemu-nbd.o qemu-io.o cmd.o qemu-ga.o: $(GENERATED_HEADERS)
 
 tools-obj-y = $(oslib-obj-y) $(trace-obj-y) qemu-tool.o qemu-timer.o \
-	qemu-timer-common.o main-loop.o notify.o iohandler.o cutils.o async.o
+	qemu-timer-common.o main-loop.o notify.o \
+	iohandler.o cutils.o iov.o async.o
 tools-obj-$(CONFIG_POSIX) += compatfd.o
 
 qemu-img$(EXESUF): qemu-img.o $(tools-obj-y) $(block-obj-y)