ArmVirtPkg: kvmtool: Add Emulated Runtime variable support

Although Kvmtool supports a CFI flash interface, it is currently
implemented using file backed support on the Host. This scenario
requires the VMM to be within the trust boundary.

In Confidential Compute Architecture the VMM is outside the trust
boundary. For such architectures Emulated Runtime variable storage
is desirable.

Therefore, make Emulated Runtime variable storage as the default
option and add a build flag ENABLE_CFI_FLASH to configure the
firmware build to use the CFI Flash as the Variable storage.

Signed-off-by: Sami Mujawar <sami.mujawar@arm.com>