qemu-ga: add guest-get-osinfo command

qemu-ga: add guest-get-osinfo command

Add a new 'guest-get-osinfo' command for reporting basic information of
the guest operating system. This includes machine architecture,
version and release of the kernel and several fields from os-release
file if it is present (as defined in [1]).

[1] https://www.freedesktop.org/software/systemd/man/os-release.html

Signed-off-by: Vinzenz Feenstra <[email protected]>
Signed-off-by: Tomáš Golembiovský <[email protected]>
* moved declarations to beginning of functions
* dropped unecessary initialization of struct utsname
Signed-off-by: Michael Roth <[email protected]>