ANDROID: KVM: arm64: Fix MMIO guard map error handling

Pierre-Clément reports that the error codes returned by the MMIO guard
map hypercall may end up being incorrectly reported as positive to
callers who interpret them a signed 64-bit integers, as specified in the
SMCCC.

Fix this by storing the return value in a 64-bit variable instead.

Bug: 253586500
Reported-by: Pierre-Clément Tosi <ptosi@google.com>
Signed-off-by: Quentin Perret <qperret@google.com>
Change-Id: I3092856ec1a1fd1648a75c9e4ad4bfebd8830d14