OvmfPkg/VmgExitLib: Add support for DR7 Read/Write NAE events

OvmfPkg/VmgExitLib: Add support for DR7 Read/Write NAE events

BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=2198

Under SEV-ES, a DR7 read or write intercept generates a #VC exception.
The #VC handler must provide special support to the guest for this. On
a DR7 write, the #VC handler must cache the value and issue a VMGEXIT
to notify the hypervisor of the write. However, the #VC handler must
not actually set the value of the DR7 register. On a DR7 read, the #VC
handler must return the cached value of the DR7 register to the guest.
VMGEXIT is not invoked for a DR7 register read.

The caching of the DR7 values will make use of the per-CPU data pages
that are allocated along with the GHCB pages. The per-CPU page for a
vCPU is the page that immediately follows the vCPU's GHCB page. Since
each GHCB page is unique for a vCPU, the page that follows becomes
unique for that vCPU. The SEC phase will reserves an area of memory for
a single GHCB and per-CPU page for use by the BSP. After transitioning
to the PEI phase, new GHCB and per-CPU pages are allocated for the BSP
and all APs.

Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Ard Biesheuvel <ard.biesheuvel@arm.com>
Acked-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Regression-tested-by: Laszlo Ersek <lersek@redhat.com>