Log Message:
qemu: Do not duplicate already existing configure nvmm test in pkgsrc Makefile

configure already had the logic to gracefully test for nvmm and if the
corresponding nvmm variable is set to an empty string do that.

No functional nor binary change intended except a CONFIGURE_ARGS difference.

Fixes PR pkg/55049.

Thanks Yasushi Oshima for reporting this problem, sharing an initial
patch and further testing the version committed now!