build-sys: simplify AF_VSOCK check

build-sys: simplify AF_VSOCK check

The current test checks more than AF_VSOCK availability, and doesn't
need to be that long.

Since its introduction in Linux in 2013, AF_VSOCK came with
linux/vm_sockets.h for sockaddr_vm, let's check that.

We could even go back to the initial configure-less approach
proposed by Stefan Hajnoczi, since Michael Roth added the configure-time
check back then to satisfy glibc in Ubuntu 14. See:
https://lists.gnu.org/archive/html/qemu-devel/2016-10/msg08208.html

Signed-off-by: Marc-André Lureau <[email protected]>
Message-Id: <20220401115005.2204000[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>