Alter qemuBuildVsockDevStr to allow passing a prefix for
the vhostfd file descriptor name. Domain startup uses
the numeric value of fd without a prefix, but hotplug
will need to use a prefix because passed file descriptor
names cannot start with a number.
https://bugzilla.redhat.com/show_bug.cgi?id=1291851
Signed-off-by: Ján Tomko <jtomko@redhat.com>
Reviewed-by: John Ferlan <jferlan@redhat.com>