Refactor kernel image loading

Let's face it: Kernel loading is quite architecture specific. Don't
claim otherwise and move the loading routines into each
architecture's responsibility.
This introduces kvm__arch_load_kernel(), which each architecture can
implement accordingly.
Provide bzImage loading for x86 and ELF loading for MIPS as special
cases for those architectures (removing the arch specific code from
the generic kvm.c file on the way) and rename the existing "flat binary"
loader functions for the other architectures to the new name.

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