qemu.tests: Update the method to get extra cpu flags

Original function can not get the whole cpu flags like tsc-deadline,
as "-" is a part of the flag. So update it. Also disable the flag
host support check for flags that want to disabled from command
line.

Signed-off-by: Yiqiao Pu <ypu@redhat.com>
Merge pull request #116 from ypu/flag_check

qemu.tests: Update the method to get extra cpu flags