Despite its name, this is really just a general-purpose string
manipulation function, so it should be moved to the virstring
module and renamed accordingly.

In addition to the obvious s/File/String/, also tweak the name
to make it clear that the presence of the suffix is verified
using case-insensitive comparison.

A few trivial whitespace changes are squashed in.
Signed-off-by: NAndrea Bolognani <abologna@redhat.com>
ACKed-by: NPeter Krempa <pkrempa@redhat.com>