Remove pci-shmem device
The pci-shmem emulated device ("ivshmem") was created by QEMU for cross-VM data sharing. The only Linux driver that uses this device is the Android Virtual System on a Chip staging driver, which also mentions a character device driver implemented on top of shmem, which was removed from Linux. On the kvmtool side, the only commits touching the pci-shmem device since it was introduced in 2012 were made when refactoring various kvmtool subsystems. Let's remove the maintenance burden on the kvmtool maintainers and remove this unused device.