mshv: Add nested virtualization creation flag

mshv: Add nested virtualization creation flag

Introduce HV_PARTITION_CREATION_FLAG_NESTED_VIRTUALIZATION_CAPABLE to
indicate support for nested virtualization during partition creation.

This enables clearer configuration and capability checks for nested
virtualization scenarios.

Signed-off-by: Stanislav Kinsburskii <skinsburskii@linux.microsoft.com>
Signed-off-by: Muminul Islam <muislam@microsoft.com>
Signed-off-by: Wei Liu <wei.liu@kernel.org>