Add machine_type option for qemu builder.

On CentOS7, ``pc-1.0'' qemu-kvm machine type are not supported.

Supported machines are:
none                 empty machine
pc                   RHEL 7.0.0 PC (i440FX + PIIX, 1996) (alias of pc-i440fx-rhel7.0.0)
pc-i440fx-rhel7.0.0  RHEL 7.0.0 PC (i440FX + PIIX, 1996) (default)
rhel6.5.0            RHEL 6.5.0 PC
rhel6.4.0            RHEL 6.4.0 PC
rhel6.3.0            RHEL 6.3.0 PC
rhel6.2.0            RHEL 6.2.0 PC
rhel6.1.0            RHEL 6.1.0 PC
rhel6.0.0            RHEL 6.0.0 PC
q35                  RHEL-7.0.0 PC (Q35 + ICH9, 2009) (alias of pc-q35-rhel7.0.0)
pc-q35-rhel7.0.0     RHEL-7.0.0 PC (Q35 + ICH9, 2009)
Merge pull request #1407 from hiroyuki-sato/patch_for_packer

builder/qemu: add machine_type option for qemu builder.