whpx: Fixed reporting of the CPU context to GDB for 64-bit

Make sure that pausing the VM while in 64-bit mode will set the
HF_CS64_MASK flag in env->hflags (see x86_update_hflags() in
target/i386/cpu.c).

Without it, the code in gdbstub.c would only use the 32-bit register values
when debugging 64-bit targets, making debugging effectively impossible.

Signed-off-by: Ivan Shcherbakov <ivan@sysprogs.com>
Message-Id: <00f701d82874$68b02000$3a106000$@sysprogs.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>