maint: consistent formatting in libvirt.c

Preliminary cleanups to make search-and-replace easier in later
patches.  Many of these were done by grepping for (multiline)
pattern violations, then bundled all into one patch.

* src/libvirt.c: Uniform two spaces between functions, return
type and open brace on separate line, avoid blank lines around
open brace, label in column 1, drop redundant (), consistent
indentation for function headers split across lines.
Signed-off-by: NEric Blake <eblake@redhat.com>