qemu: Forbid "sgio" support for SCSI generic host device

qemu: Forbid "sgio" support for SCSI generic host device

The kernel didn't support the unprivileged SGIO for SCSI generic
device finally, and since it's unknow whether the way to support
unprivileged SGIO for SCSI generic device will be similar as for
SCSI block device or not, even it's simliar (I.e. via sysfs, for
SCSI block device, it's /sys/dev/block/8\:0/queue/unpriv_sgio,
for example), the file name might be different, So it's better not
guess what it should be like currently.

This patch removes the related code (mainly about the "shareable"
checking on the "sgio" setting, it's not supported at all, why
we leave checking code there? :-), and error out if "sgio" is
specified in the domain config.