This problem was fixed in Xamarin Studio 5.8 ac846289
and seems to have re-surfaced. On updating Xamarin.Forms in a solution
that contains multiple projects that use it the build would sometimes
fail with errors like:

    Could not load file or assembly 'Xamarin.Forms.Xaml,
    Version=2.0.0.0, Culture=neutral, PublicKeyToken=null' or one of
    its dependencies.

    Error XF002: Xamarin.Forms tasks do not match targets. Please
    ensure that all projects reference the same version of
    Xamarin.Forms, and if the error persists, please restart the IDE.

The old MSBuild tasks and targets were being used after they have
been updated. Now when an MSBuild import has changed in a project
the remote MSBuild host is shutdown to ensure the correct tasks are
used.

Note that currently .NET Core projects are not fixed with this change
since they have no MSBuild imports in the project.

Fixes VSTS #576301 - Getting "Could not load file or assembly
'Xamarin.Forms.Xaml" when update forms version to prerelease and build
project
Fixes VSTS #571579 - Reopen solution needed after updating Xamarin.Forms
package