OvmfPkg/AmdSevDxe: Close mAcceptAllMemoryEvent

OvmfPkg/AmdSevDxe: Close mAcceptAllMemoryEvent

This event should only trigger once. It should be idempotent, but the
allocation of the memory map itself is observable and can cause
ExitBootServices to fail with a modified map key.

Cc: Thomas Lendacky <Thomas.Lendacky@amd.com>
Cc: Erdem Aktas <erdemaktas@google.com>
Cc: James Bottomley <jejb@linux.ibm.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Min Xu <min.m.xu@intel.com>
Cc: Michael Roth <michael.roth@amd.com>
Signed-off-by: Dionna Glaze <dionnaglaze@google.com>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>