virFileOpenAs takes desired uid:gid as arguments, and not only uses
them for a fork/setuid/setgid when retrying failed open operations,
but additionally always forces the opened file to be owned by the
given uid:gid.

One example of the problems this causes is that, when restoring a
domain from a file that is owned by the qemu user, opening the file
chowns it to root. if dynamic_ownership=1 this is coincidentally
expected, but if dynamic_ownership=0, no existing file should ever
have its ownership changed.

This patch adds an extra check before calling fchown() - it only does
it if O_CREAT was passed to virFileOpenAs() in the openflags.