library/*: Use short form for target definitions

There are currently two ways to specify the platform and architecture in
`Kraftfile`s with rules to build / run targets:

a)

```
- plat: qemu
  arch: arm64
```

b)

```
- qemu/arm64
```

For simplicity and conciseness, go for option b) in all `Kraftfile`s
that specify targets without target-specifics configs.

Signed-off-by: Razvan Deaconescu <razvand@unikraft.io>

library/helloworld/Kraftfile

@@ -5,15 +5,9 @@ name: helloworld
 unikraft: stable
 
 targets:
-- plat: qemu
- arch: x86_64
-- plat: qemu
- arch: arm64
-- plat: fc
- arch: x86_64
-- plat: xen
- arch: x86_64
-- plat: linuxu
- arch: x86_64
-- plat: linuxu
- arch: arm64
+- qemu/x86_64
+- qemu/arm64
+- fc/x86_64
+- xen/x86_64
+- linuxu/x86_64
+- linuxu/arm64

library/nginx1.15/Kraftfile

@@ -38,14 +38,10 @@ unikraft:
  CONFIG_VIRTIO_PCI: 'y'
 
 targets:
-- plat: qemu
- arch: x86_64
-- plat: qemu
- arch: arm64
-- plat: fc
- arch: x86_64
-- plat: fc
- arch: arm64
+- qemu/x86_64
+- qemu/arm64
+- fc/x86_64
+- fc/arm64
 
 libraries:
  musl: staging

library/python3.10/Kraftfile

@@ -33,19 +33,14 @@ unikraft:
  CONFIG_STACK_SIZE_PAGE_ORDER: 10
  CONFIG_UKSYSINFO: 'y'
  CONFIG_VIRTIO_PCI: 'y'
+ CONFIG_KVM_PCI: 'y'
  # Debug options
  # CONFIG_LIBUKDEBUG_PRINTD: 'y'
  # CONFIG_LIBUKDEBUG_PRINTK_INFO: 'y'
 
 targets:
-- plat: qemu
- arch: x86_64
- kconfig:
- CONFIG_KVM_PCI: 'y'
-- plat: fc
- arch: x86_64
- kconfig:
- CONFIG_KVM_PCI: 'y'
+- qemu/x86_64
+- fc/x86_64
 
 libraries:
  musl: staging