OvmfPkg/QemuFlashFvbServicesRuntimeDxe: Fix erase blocks for SEV-ES

OvmfPkg/QemuFlashFvbServicesRuntimeDxe: Fix erase blocks for SEV-ES

BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=3008

The original SEV-ES support missed updating the QemuFlashEraseBlock()
function to successfully erase blocks. Update QemuFlashEraseBlock() to
call the QemuFlashPtrWrite() to be able to successfully perform the
commands under SEV-ES.

Fixes: 437eb3f7a8db7681afe0e6064d3a8edb12abb766
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Ard Biesheuvel <ard.biesheuvel@arm.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Message-Id: <309c5317a3107bd0e650be20731842a2e1d4b59a.1604685192.git.thomas.lendacky@amd.com>