pvg: do not enable it on cross-architecture targets

pvg: do not enable it on cross-architecture targets

PVG is not cross-architecture; the PVG guest drivers with x86-64 macOS do not give
useful results with the aarch64 macOS host PVG framework, and vice versa.
To express this repurpose CONFIG_MAC_PVG, making it true only if the target has
the same architecture as the host.  Furthermore, remove apple-gfx.m unless
one of the devices is actually present.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>