iscsi: send readcapacity10 when readcapacity16 failed

iscsi: send readcapacity10 when readcapacity16 failed

When play with Dell MD3000 target, for sure it
is a TYPE_DISK, but readcapacity16 would fail.
Then we find that readcapacity10 succeeded. It
looks like the target just support readcapacity10
even through it is a TYPE_DISK or have some
TYPE_ROM characteristics.

This patch can give a chance to send
readcapacity16 when readcapacity10 failed.
This patch is not harmful to original pathes

Signed-off-by: Zhu Lingshan <lszhu@suse.com>
Message-Id: <1451359934-9236-1-git-send-email-lszhu@suse.com>
[Don't fall through on UNIT ATTENTION. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>