kvm-unit-tests: VMX: Fix confused definition of rflags

Change "rflags" in "struct regs" to "host_rflags". Remove settings
to GUEST_RFLAGS since GUEST_RFLAGS can be set by vmwrite. Treat
host_rflags as host rflags before and after vmenter.

Besides, add checks to flags after vmenter.

Signed-off-by: Arthur Chunqi Li <yzt356@gmail.com>