generic.tests.pxe: recorrect param for pxe test

Signed-off-by: Xu Tian <xutian@redhat.com>
qemu.tests.pxe_query_cpus: small enhancement

Signed-off-by: Xu Tian <xutian@redhat.com>
qemu.generic.tests: rename pxe test to pxe_boot test

we have pxe installation test and pxe boot test, which made user
feeling confused. so rename pxe test to pxe_boot test.

Signed-off-by: Xu Tian <xutian@redhat.com>
Merge pull request #44 from xutian/fix_pxe_test

small fix for pxe test