efi_loader: enable QueryVariableInfo at runtime for file backed variables

Since commit c28d32f9 ("efi_loader: conditionally enable SetvariableRT")
we are enabling the last bits of missing runtime services.
Add support for QueryVariableInfo which we already support at boottime
and we just need to mark some fucntions available at runtime and move
some checks around.

It's worth noting that pointer checks for maxmimum and remaining
variable storage aren't when we store variables on the RPMB, since the
Secure World backend is already performing them.

Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>