nbd: produce a better error if neither host nor port is passed

Before:
    $ qemu-io-old
    qemu-io-old> open -r -o file.driver=nbd
    qemu-io-old: can't open device (null): Could not open image: Invalid argument
    $ ./qemu-io-old
    qemu-io-old> open -r -o file.driver=nbd,file.host=foo,file.path=bar
    path and host may not be used at the same time.
    qemu-io-old: can't open device (null): Could not open image: Invalid argument

After:
    $ ./qemu-io
    qemu-io> open -r -o file.driver=nbd
    one of path and host must be specified.
    qemu-io: can't open device (null): Could not open image: Invalid argument
    $ ./qemu-io
    qemu-io> open -r -o file.driver=nbd,file.host=foo,file.path=bar
    path and host may not be used at the same time.
    qemu-io: can't open device (null): Could not open image: Invalid argument

Next patch will fix the error propagation.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
