kvm-ipc: use proper type for file descriptor

A socket (as any other file descriptor) is of type "int" to catch the
negative error cases. Fix the declaration to allow errors to be
detected.
Found and needed by clang.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>