Signed-off-by: Wei Liu <liuwe@microsoft.com>

Rename KvmSetDeviceAttr to SetDeviceAttr and modify private helper
functions to work with both hypervisors.

Signed-off-by: Wei Liu <liuwe@microsoft.com>

Generally speaking functions take RawFd should be marked as unsafe.
device_add_group and device_del group are such functions.

Instead of marks them as unsafe, we can make these two functions take a
reference to VfioGroup. This is far more elegant.

Signed-off-by: Wei Liu <liuwe@microsoft.com>

Rust does not support mutually exclusive features. Buildkite enables all
features. These two factors combined causes pipeline failures.

Turn the feature gates in code a bit. Whenever "kvm" is specify, surface
KVM's definitions. Make available MSHV if and only if "mshv" is
specified.

Add custom tests for MSHV.

Signed-off-by: Wei Liu <liuwe@microsoft.com>