qemu.tests.hypervisor_flag: test for 'hypervisor' flag

Sanity check for 'hypervisor' flag in cpuinfo for the running guest VM.

Signed-off-by: Ruda Moura <rmoura@redhat.com>
Merge pull request #1269 from ruda/hypervisor-flag

qemu.tests.hypervisor_flag: test for 'hypervisor' flag