qemu-iotests: limit non-_PROG-suffixed variables to common.rc

These are never used by "check", with one exception that does not need
$QEMU_OPTIONS.  Keep them in common.rc, which will be soon included only
by the tests.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
