qemu-config: add error propagation to qemu_config_parse

This enables some simplification of vl.c via error_fatal, and improves
error messages.  Before:

  $ ./qemu-system-x86_64 -readconfig .
  qemu-system-x86_64: error reading file
  qemu-system-x86_64: -readconfig .: read config .: Invalid argument
  $ /usr/libexec/qemu-kvm -readconfig foo
  qemu-kvm: -readconfig foo: read config foo: No such file or directory

After:

  $ ./qemu-system-x86_64 -readconfig .
  qemu-system-x86_64: -readconfig .: Cannot read config file: Is a directory
  $ ./qemu-system-x86_64 -readconfig foo
  qemu-system-x86_64: -readconfig foo: Could not open 'foo': No such file or directory

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20210226170816.231173-1-pbonzini@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>