Give ivshmem its own config option

Give ivshmem its own config option

Currently the ivshmem device is built whenever both PCI and KVM support are
included.  This patch gives it its own config option to allow easier
customization of whether to include it.  It's enabled by default in the
same circumstances as now - when both PCI and KVM are available.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Message-Id: <1425017077-18487-4-git-send-email-david@gibson.dropbear.id.au>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>