avoid casts when initializing structures

Due to our kernel heritage we have code in kvmtool that relies on
the (still) implicit -std=gnu89 compiler switch.
It turns out that this just affects some structure initialization,
where we currently provide a cast to the type, which upsets GCC for
anything beyond -std=gnu89 (for instance gnu99 or gnu11).
We do need the casts when initializing structures that are not
assigned to the same type, so we put it there explicitly.

This allows us to compile with all the three GNU standards GCC
currently supports: gnu89/90, gnu99 and gnu11.
GCC threatens people with moving to gnu11 as the new default standard,
so lets fix this better sooner than later.
(Compiling without GNU extensions still breaks and I don't bother to
fix that without very good reasons.)

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