rename qemu_sendv to iov_send, change proto and move declarations to iov.h

Rename arguments and use size_t for sizes instead of int,
from
 int
 qemu_sendv(int sockfd, struct iovec *iov,
            int len, int iov_offset)
to
 ssize_t
 iov_send(int sockfd, struct iovec *iov,
          size_t offset, size_t bytes)

The main motivation was to make it clear that length
and offset are in _bytes_, not in iov elements: it was
very confusing before, because all standard functions
which deals with iovecs expects number of iovs, not
bytes, even the fact that struct iovec has iov_len and
iov_ prefix does not help.  With "bytes" and "offset",
especially since they're now size_t, it is much more
explicit.  Also change the return type to be ssize_t
instead of int.

This also changes it to match other iov-related functons,
but not _quite_: there's still no argument indicating
where iovec ends, ie, no iov_cnt parameter as used
in iov_size() and friends.  If will be added in subsequent
patch/rewrite.

All callers of qemu_sendv() and qemu_recvv() and
related, like qemu_co_sendv() and qemu_co_recvv(),
were checked to verify that it is safe to use unsigned
datatype instead of int.

Note that the order of arguments is changed to: offset
and bytes (len and iov_offset) are swapped with each
other.  This is to make them consistent with very similar
functions from qemu_iovec family, where offset always
follows qiov, to mean the place in it to start from.

Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
