arm,arm64: do not always merge biovec if we are running on Xen

This is similar to what it is done on X86: biovecs are prevented from merging
otherwise every dma requests would be forced to bounce on the swiotlb buffer.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>

Changes in v7:
- remove the extra autotranslate check in biomerge.c.
Git-commit: 3d1975b57097820c131c692d2e0d075641370369
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


[imaund@codeaurora.org: resolve merge conflicts]
Signed-off-by: Ian Maund <imaund@codeaurora.org>