virt-host-validate: Turn failure to read /proc/cmdline into an error
virt-host-validate: Turn failure to read /proc/cmdline into an error

When validating secure guests support on s390(x) we may read
/proc/cmdline and look for "prot_virt" argument. Reading the
kernel command line is done via virFileReadValueString() which
may fail. In such case caller won't see any error message. But we
can produce the same warning/error as if "prot_virt" argument
wasn't found.  Not only this lets users know about the problem,
it also terminates the "Checking for ...." line correctly.

Signed-off-by: Michal Privoznik <83d82aaba2eed257f4814b0c239c260c4caaadf0@redhat.com>
Reviewed-by: Ján Tomko <4cab11cfb98d3c937327354a78eb07dbb6ee2bc6@redhat.com>
Reviewed-by: Fabiano Fidêncio <d95e9796398a6087d3fb67947e5776c26b264733@fidencio.org>

jfehlig/libvirt,libvirt/libvirt,olafhering/libvirt,libvirt/libvirt,nertpinx/libvirt,zippy2/libvirt,zippy2/libvirt,nertpinx/libvirt,crobinso/libvirt,nertpinx/libvirt,crobinso/libvirt,zippy2/libvirt,libvirt/libvirt,olafhering/libvirt,olafhering/libvirt,nertpinx/libvirt,libvirt/libvirt,crobinso/libvirt,crobinso/libvirt,olafhering/libvirt,jfehlig/libvirt,jfehlig/libvirt,zippy2/libvirt,jfehlig/libvirt,nertpinx/libvirt