qemu.tests.virtual_nic_send_buffer: Update the way used to checkout md5 in guest

md5 command in guest may have different output format. It is a little difficult to
filter out the md5 value from the output. So just checkout whether correct md5 value
is in the output is enough.

Signed-off-by: Feng Yang <fyang@redhat.com>
Merge pull request #1290 from FengYang/virtual_nic_send_buffer

qemu.tests.virtual_nic_send_buffer: Update the way used to checkout md5 ...