qemu-sockets: add error propagation to Unix socket functions

qemu-sockets: add error propagation to Unix socket functions

Before:

    $ qemu-system-x86_64 -monitor unix:/vvv,server=off
    connect(unix:/vvv): No such file or directory
    chardev: opening backend "socket" failed

After:

    $ x86_64-softmmu/qemu-system-x86_64 -monitor unix:/vvv,server=off
    qemu-system-x86_64: -monitor unix:/vvv,server=off: Failed to connect to socket: No such file or directory
    chardev: opening backend "socket" failed

Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>