util: introduce qemu_open and qemu_create with error reporting

util: introduce qemu_open and qemu_create with error reporting

qemu_open_old() works like open(): set errno and return -1 on failure.
It has even more failure modes, though.  Reporting the error clearly
to users is basically impossible for many of them.

Our standard cure for "errno is too coarse" is the Error object.
Introduce two new helper methods:

  int qemu_open(const char *name, int flags, Error **errp);
  int qemu_create(const char *name, int flags, mode_t mode, Error **errp);

Note that with this design we no longer require or even accept the
O_CREAT flag. Avoiding overloading the two distinct operations
means we can avoid variable arguments which would prevent 'errp' from
being the last argument. It also gives us a guarantee that the 'mode' is
given when creating files, avoiding a latent security bug.

Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>