test: qemuxml2argv: Mock virMemoryMaxValue to remove 32/64 bit difference

test: qemuxml2argv: Mock virMemoryMaxValue to remove 32/64 bit difference

Always return LLONG_MAX even on 32 bit systems. The limitation
originates from our use of "unsigned long" in several APIs. The internal
data type is unsigned long long. Make the test suite deterministic by
removing the architecture difference.

Flaw was introduced in 645881139b3d2c86acf9d644c3a1471520bc9e57 where
I've added a test that uses too large numbers.