This patch makes necessary integer narrowing and/or signedness conversion
explicit.
While some narrowing are expected behavior, the correctness of the others relies
on the specifications of some interfaces (e.g. the higher 32-bit of what
exec_vmread() returns is all 0s if the given field is 32-bit). Add a stub
for now to avoid missing them.
Signed-off-by: Junjie Mao <junjie.mao@intel.com>