kvmtool: remove warning about bzImage on non-x86 architectures

Among the architectures supported by kvmtool, only x86 defines a
bzImage format. So we shouldn't bother users of other architectures
with a message about something that cannot work.
Make the bzImage check dependent on compiling for x86.

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