exec: check that alignment is a power of two

exec: check that alignment is a power of two

Right now we can crash QEMU using e.g.

qemu-system-x86_64 -m 256M,maxmem=20G,slots=2 \
 -object memory-backend-file,id=mem0,size=12288,mem-path=/dev/zero,align=12288 \
 -device pc-dimm,id=dimm1,memdev=mem0

qemu-system-x86_64: util/mmap-alloc.c:115:
 qemu_ram_mmap: Assertion `is_power_of_2(align)' failed

Fix this by adding a proper check.

Signed-off-by: David Hildenbrand <[email protected]>
Message-Id: <20180607154705[email protected]>
Reviewed-by: Michael S. Tsirkin <[email protected]>
Reviewed-by: Igor Mammedov <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>