Fix error handling in qemu_read_config_file

We need to close the file even in error case. While at it, make the callers
catch all kind of errors. ENOENT is allowed for default config files, they
are optional.

Reported-by: Luiz Capitulino <lcapitulino@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>