virt-admin: Properly fix the default session daemon URI to admin server

virt-admin: Properly fix the default session daemon URI to admin server

Commit 30ce2f0e tried to fix the issue with an incorrect session URI to admin
server but it messed up the checks:

    if (geteuid == 0 && VIR_STRDUP(*uristr, "libvirtd:///system") < 0)
        return -1;
    else if (VIR_STRDUP(*uristr, "libvirtd:///session") < 0)
        return -1;

So if a client executed with root privileges tries to connect, its euid is
checked (true) and the correct URI is successfully copied to @uristr (false),
therefore the 'else' branch is taken and @uristr is replaced by the session URI
which for root results in:
    Failed to connect socket to '/root/.cache/libvirt/libvirt-admin-sock':
    No such file or directory

Signed-off-by: Erik Skultety <eskultet@redhat.com>