Turn 1<<31 into 1U<<31

Turn 1<<31 into 1U<<31

Apparently, 1 << 31 is signed which in turn does not fit into
a signed integer variable:

../../include/libvirt/libvirt-domain.h:1881:57: error: result of '1 << 31' requires 33 bits to represent, but 'int' only has 32 bits [-Werror=shift-overflow=]
     VIR_CONNECT_GET_ALL_DOMAINS_STATS_ENFORCE_STATS = 1 << 31, /* enforce requested stats */
                                                         ^~
cc1: all warnings being treated as errors

The solution is to make it an unsigned value. I've found only two
such occurrences in our code base.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>