insufficient grant unmapping checks for x86 PV guests

ISSUE DESCRIPTION When removing or replacing a grant mapping, the x86 PV specific path needs to make sure page table entries remain in sync with other accounting done. Although the identity of the page frame was validated correctly, neither the presence of the mapping nor page writability were...

linux: Fix Xen block IO merge-ability calculation

ISSUE DESCRIPTION The block layer in Linux may choose to merge adjacent block IO requests. When Linux is running as a Xen guest, the default merging algorithm is replaced with a Xen-specific one. When Linux is running as an x86 PV guest, some BIO's are erroneously merged, corrupting the data stre...