virFileRewrite: Allow setting owner

virFileRewrite: Allow setting owner

Currently, due to the way virFileRewrite() works, the rewritten
file is owned by user and group that the daemon runs under. So
far, this is not a problem, because the function is used to write
XML files or secrets for persistent objects (domains, networks,
etc.) and we don't need other users to read/write those files.

But shortly, this function is going to be used for creating files
for QEMU domains. There we want the QEMU process (i.e. different
user) to read the file.

Therefore, introduce two new arguments: @uid and @gid that allow
setting desired owner of the file. Pass -1 to preserve current
behaviour (i.e. create the file owned by the user running the
daemon).

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>