Fix parsing of uid/gid on Mingw32

Fix parsing of uid/gid on Mingw32

The DAC security driver uses the virStrToLong_ui function to
parse the uid/gid out of the seclabel string. This works on
Linux where 'uid_t' is an unsigned int, but on Mingw32 it is
just an 'int'. This causes compiler warnings about signed/
unsigned int pointer mis-match.

To avoid this, use explicit 'unsigned int ouruid' local
vars to pass into virStrToLong_ui, and then simply assign
to the 'uid_t' type after parsing

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>