lockable: add QemuLockable

lockable: add QemuLockable

QemuLockable is a polymorphic lock type that takes an object and
knows which function to use for locking and unlocking.  The
implementation could use C11 _Generic, but since the support is
not very widespread I am instead using __builtin_choose_expr and
__builtin_types_compatible_p, which are already used by
include/qemu/atomic.h.

QemuLockable can be used to implement lock guards, or to pass around
a lock in such a way that a function can release it and re-acquire it.
The next patch will do this for CoQueue.

Signed-off-by: Paolo Bonzini <[email protected]>
Message-Id: <20180203153935[email protected]>
Reviewed-by: Richard Henderson <[email protected]>
Reviewed-by: Stefan Hajnoczi <[email protected]>
Reviewed-by: Fam Zheng <[email protected]>
Signed-off-by: Fam Zheng <[email protected]>