spec: Disable libssh2 support for RHEL

https://bugzilla.redhat.com/show_bug.cgi?id=905513

Libssh2 isn't reliable enough to support the libvirt transport using it.
The problems include mishandling of "known_hosts" files that may confuse
users.

libvirt.spec.in

@@ -255,7 +255,7 @@
 %endif
 
 # Enable libssh2 transport for new enough distros
-%if 0%{?fedora} >= 17 || 0%{?rhel} >= 6
+%if 0%{?fedora} >= 17
     %define with_libssh2 0%{!?_without_libssh2:1}
 %endif