Commit 843a70a8 changed test-wrap-argv.pl to use
/usr/bin/env perl
instead of
/usr/bin/perl

However when called from qemuxml2argvtest with
VIR_TEST_REGENERATE_OUTPUT, PATH is set to '/bin'.

Find the path to perl early in virTestMain, in case we
are going to need it later after we've overridden PATH.