Restore cr3 after tests on PCID

The INVPCID enabling test assumes cr3[11:0] is 0. But at present PCID enabling
test sets cr3[11:0] to 1 for its own purpose and doesn't restore the register,
which leads to a failure when INVPCID test tries to enable PCIDE.

This patch restores cr3 after PCID enabling test is done so that PCIDE can be
enabled normally in later tests.

Signed-off-by: Junjie Mao <junjie.mao@intel.com>
Signed-off-by: Avi Kivity <avi@redhat.com>