virSetUIDGIDWithCaps: Set bounding capabilities only with CAP_SETPCAP

virSetUIDGIDWithCaps: Set bounding capabilities only with CAP_SETPCAP

In one of my previous patches I've tried to postpone dropping
CAP_SETPCAP until the very end because it's needed for
capng_apply(). What I did not realize back then was that we might
not have the capability to begin with. Because of unknown reasons
capng_apply() pollutes logs only for CAPNG_SELECT_BOUNDS and not
for CAPNG_SELECT_CAPS.

Reproducer is really simple: run libvirtd as a regular user.
During its initialization, libvirtd will spawn some binaries
(dnsmasq, qemu-*, etc.) and while doing so it will try to drop
capabilities.

Anyway, let's call capng_apply(CAPNG_SELECT_BOUNDS) only if we
have the CAP_SETPCAP (which is tracked in need_setpcap variable).

Fixes: 438b50dda8a863fdc988e9ab612f097cc1626e8a
Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1924218
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Cole Robinson <crobinso@redhat.com>