exec: round up size on MR resize

exec: round up size on MR resize

Block size must fundamentally be a multiple of target page size.
Aligning automatically removes need to worry about the alignment
from callers.

Note: the only caller of qemu_ram_resize (acpi) already happens to have
size padded to a power of 2, but we would like to drop the padding in
ACPI core, and don't want to expose target page size knowledge to ACPI.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Paolo Bonzini <ponzini@redhat.com>