libxl: Enhance libxl__sendmsg_fds to deal with EINTR and EWOULDBLOCK

libxl: Enhance libxl__sendmsg_fds to deal with EINTR and EWOULDBLOCK

This patch change the behavior of libxl__sendmsg_fds to retry sendmsg on
EINTR error and return an error on short writes.

This patch allow a caller of libxl__sendmsg_fds to deal with EWOULDBLOCK
and short writes. The function now requires to send only 1 byte of data
so that when dealing with non-blocking fds a EWOULDBLOCK error would
mean that the fds haven't been sent yet. Current caller already send
only 1 byte.

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
[ wei: fix build ]
Signed-off-by: Wei Liu <wei.liu2@citrix.com>