tests/util/grub-shell: Default qemuopts to envvar $GRUB_QEMU_OPTS

tests/util/grub-shell: Default qemuopts to envvar $GRUB_QEMU_OPTS

Fix a regression where qemuopts was mistakenly defaulted to the empty
string. This prevents the sending of arbitrary QEMU options to tests,
which may be desirable for overriding the machine type. There was a
concern that allowing the tester to accept arbitrary options would add
headaches for another developer trying to diagnose why a test failed on
the testers machine because he could not be sure if any additional
options were passed to make the test fail. However, all the options are
recorded in the run.sh generated script, so this concern is unwarranted.

Fixes: 6d729ced70 (tests/util/grub-shell: Add $GRUB_QEMU_OPTS to run.sh to easily see unofficial QEMU arguments)
Signed-off-by: Glenn Washburn <development@efficientek.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>