ANDROID: BACKPORT: KVM: arm64: Add initial support for KVM_CAP_EXIT_HYPERCALL

Allow the VMM to hook into and handle a subset of guest hypercalls
advertised by the host. For now, no such hypercalls exist, and so the
new capability returns 0 when queried.

Signed-off-by: Will Deacon <will@kernel.org>
Bug: 209580772
Change-Id: I684d5cd07864887377e91cc96041916d671b2b16
[willdeacon@: Leave other unsupported CAPs commented in uapi/linux/kvm.h]
Signed-off-by: Will Deacon <willdeacon@google.com>