scsi-generic: Add share-rw option

Add the property to the device model, then parse it by calling
blkconf_apply_backend_options().

In addition to blk_set_perm(), the called function also handles error
options and wce. For error options we've already checked that the
default values are used, for wce we don't have the option either so it
is always the default (true). In other words there is no change of
behavior in these regards.

Signed-off-by: Fam Zheng <famz@redhat.com>
Message-Id: <20171205151553.7834-1-famz@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>