qemu.test.virtual_nic_send_buffer: Some modification

Make file transfer support tcp, udp or both.
Add netperf_udp burst test
ignore errors when deleting the test file on host and guest.

Signed-off-by: Yunping Zheng <yunzheng@redhat.com>
Merge pull request #843 from humanux/virtual_nic_send_buffer

Virtual nic send buffer