vmm: added unit tests in vstate.rs
vmm: added unit tests in vstate.rs

Merged the tests related to VCPUs in one test called
test_vcpu_configure to remove duplicated lines of code.

Increased coverage for vstate.rs from 36.7% to 76.5%.

Signed-off-by: Andreea Florescu <f68a7d732e796861f8e79ea704b43a5ee52eddaf@amazon.com>

firecracker-microvm/firecracker,firecracker-microvm/firecracker,firecracker-microvm/firecracker,firecracker-microvm/firecracker,firecracker-microvm/firecracker