[virtio-pmem] Add support for virtio-pmem device

[ShadowCurse]

This issue tracks the development of the support of the virtio-pmem device in Firecracker.
virtio-pmem is a visualized persistent memory device which is backed by the memory mapped file on the host and presented to the guest as a memory region. This allows the guest to directly read/write to the underlying file without driver/VMM intervention and without guest page cache usage.

Firecracker support for this device will include:

• Ability to specify virtio-pmem device visible to the guest
• Ability to configure virtio-pmem device to be a root device, so VMs will be able to boot from this device
• Ability to configure virtio-pmem device to mmap the underlying file as read only

All additional details and usage recommendations will be included in the accompanying documentation.