qemu: alias: Generate 'qomName' of disk with useraliases

qemu: alias: Generate 'qomName' of disk with useraliases

Commit fb64e176f4f forgot to delete the check that short-circuits the
disk alias creation if the alias is already present. The side effect
of this is that the creation qomName which is necessary to be able to
refer to disk frontends when -blockdev is used was skipped when user
aliases are used.

Fix it by deleting the check. Also prevent any potential memory leaks
from calling this function repeatedly by creating the qomName only when
it's not present.

https://bugzilla.redhat.com/show_bug.cgi?id=1741838

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>