vmmdev: return EFAULT when trying to read beyond VM system memory max address

Currently, when using dd(1) to take a VM memory image, the capture never ends,
reading zeroes when it's beyond VM system memory max address.
Return EFAULT when trying to read beyond VM system memory max address.

Reviewed by: imp, grehan, anish
Approved by: grehan
Differential Revision: https://reviews.freebsd.org/D15156