When OVMF is built with the SECURE_BOOT_ENABLE set to true, reserving and
initializing the emulated variable store happens before memory has been
accepted under SEV-SNP. This results in a #VC exception for accessing
memory that hasn't been validated (error code 0x404). The #VC handler
treats this error code as a fatal error, causing the OVMF boot to fail.
Move the call to ReserveEmuVariableNvStore() to after memory has been
accepted by AmdSevInitialize().
Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>