KVM: arm64: Separate out feature sanitisation and initialisation

kvm_vcpu_set_target() iteratively sanitises and copies feature flags in
one go. This is rather odd, especially considering the fact that bitmap
accessors can do the heavy lifting. A subsequent change will make vCPU
features VM-wide, and fitting that into the present implementation is
just a chore.

Rework the whole thing to use bitmap accessors to sanitise and copy
flags.

Link: https://lore.kernel.org/r/20230609190054.1542113-2-oliver.upton@linux.dev


Signed-off-by: Oliver Upton <oliver.upton@linux.dev>