qemu.tests: Add machine_type check in physical_resource_check

Add machine_type check function to check the guest's machine type

change from v1:
1. change 'extra_params' to 'machine type'
2. add try except block when compare the result

Signed-off-by: Golita Yue <gyue@redhat.com>
qemu.tests: Add serial name check in physical_resources_check

Improve physical_resources_check in serial name part.
  - Add a random serial name to guest less than 20 characters
  - Add strict check for serial name

changes from v1:
  - format modify
changes from v2:
  - move randome generate string to virt_utils
changes from v3:
  - move this test only for virtio_blk and only for RHEL 6 guest
    as test plan
changes from v4:
  - clean up and make this test will not run in Windows guests

Signed-off-by: Yiqiao Pu <ypu@redhat.com>
Acked-by: Feng Yang <fyang@redhat.com>
Acked-by: Qingtang Zhou <qzhou@redhat.com>